Numerous airline executives say no, but a growing number of politicians and medical experts say it is worth following Canada’s lead before holiday travel commences.By the holiday season, flying will have changed dramatically for Americans returning to the United States from abroad. They will be asked to show proof that they are vaccinated, to commit to two coronavirus tests if they are not and to participate in a new contact tracing system.For Americans traveling within the United States, however, none of this applies. As airlines prepare for what’s expected to be the biggest travel rush of the past two years, domestic travel — aside from a mask mandate and some restrictions on alcohol — will be largely the same as it was before the pandemic: packed cabins and no testing or proof of vaccination required.Whether this is a symptom of denial or a sign of progress depends on who you ask. Senator Dianne Feinstein, Democrat of California, recently telegraphed her position when she proposed a bill that would require passengers on domestic flights to be fully vaccinated, to have recently tested negative or to have a certificate of recovery from the virus.“We know that air travel during the 2020 holiday season contributed to last winter’s devastating COVID-19 surge,” Ms. Feinstein said in a statement. “We simply cannot allow that to happen again.”Dr. Anthony S. Fauci, President Biden’s top medical adviser, has said that he personally supports the idea of requiring proof of vaccination for domestic air travel, a policy that Canada will begin implementing on Oct. 30. The White House has said that it is focused on other strategies for encouraging broad vaccination.Requiring proof of vaccination to fly is ludicrous say numerous airline executives and representatives of airline trade organizations, who tend to reference a study that found, by some measures, that flight cabin air is “safer than the air in a surgical operating room.” Most Americans, they add, are vaccinated and the existing federal mask mandate should radically reduce any lingering risk of transmission. Plus virtually all crew members, they say, will soon be inoculated in compliance with President Biden’s vaccine requirements for government contractors and many airlines’ own requirements for employees.“We don’t see any reason to mandate vaccination in the domestic market,” said Willie Walsh, the director general of the International Air Transport Association, an airline trade group, at the World Air Transport Summit, a gathering last week of hundreds of airline executives in Boston. If you mandate vaccination and testing for flights, then you need to mandate it for all forms of transport, Mr. Walsh said, because in terms of the likelihood of transmitting coronavirus, “a plane is less risky than a train or a bus or a car.”Such a requirement would also create even longer airport lines and other organizational nightmares, said Robin Hayes, chief executive of JetBlue Airways.But if small restaurants in New York City can figure out how to check proof of vaccination without undermining the dining experience, then certainly the airline industry can work something out, said Dr. Georges Benjamin, executive director of the American Public Health Association, a health professionals association that supports Ms. Feinstein’s bill. Yes, the risk of transmission on planes is low, but only if everyone is properly wearing masks during the entire flight, which is far from guaranteed, he said. Neither do we understand, as several researchers have noted, how the Delta variant — or other variants — have changed the likelihood of spreading the virus on planes.Parsing the “it’s-too-complicated” argumentIn order to attend the first in-person World Air Transport Summit in two years, held at the Boston Park Plaza Hotel, journalists and airline industry executives had to show proof of vaccination , the negative results of a recent coronavirus test or a certificate of recovery. Attendees were supposed to wear a mask indoors except when eating, drinking or speaking on a panel.Though every room seemed to have at least four people flouting the conference’s mask mandate, no one queried seemed to object to the vaccine or testing requirement. When asked why domestic airlines should not also have these rules, Mr. Hayes of JetBlue Airways explained that it’s different because, while the conference had a few hundred attendees, roughly three million people are once again flying domestically every day. Though he supports encouraging vaccination, requiring proof from all those people involves too much “operational complexity,” he said.Unlike Canada, Israel and the European Union, the United States has not created a uniform digital proof of vaccination or coronavirus status system. That means that airlines and airports would potentially have to come up with a whole new way of reviewing and verifying results. Domestic travelers eager to check in online and whiz through security “would have to come earlier,” Mr. Hayes said. “There would be longer lines.”Doug Parker, the chief executive of American Airlines, made a similar comment in an interview on the Sway podcast in August,The White House chief of staff, Ron Klain, also expressed concerns that a vaccine mandate would create longer airport lines on the Pod Save America show last month, as the Washington Post previously reported. Mr. Klain also suggested that though such a policy is “something we continue to look at,” it might be unnecessary given that the administration is pushing vaccination through employer and military vaccination mandates.Airlines for America, an industry trade group that represents eight airlines shares the administration’s concerns about the implementation of a domestic travel requirement Katherine Estep, a spokeswoman for the group, said last week.Leonard J. Marcus, the co-director of the National Preparedness Leadership Initiative at Harvard University and the director of an initiative focused on public health on flights, said that airlines are not exaggerating when they say it’s complicated. But he thinks they are wrong to dismiss a domestic testing and vaccination requirement for flying because a good system does not yet exist. “I do believe we should get to the point where we have those mechanics,” he said.Dr. Patrice Harris, the former president of the American Medical Association and the chief executive of eMed, which sells at-home coronavirus tests to travelers and Delta Air Lines, said the “it’s-too-complicated” argument is a poor excuse. “The key is making the right thing to do, the easy thing to do,” she said.Hawaii is the one state that requests proof of vaccination or a negative test from domestic visitors. In order to avoid quarantine, travelers can upload an image of their C.D.C.-issued vaccination card or a negative coronavirus test to the state’s Safe Travels system. The system has gone through several iterations: Early on, checking test results created terrible lines at airports and caused havoc for travelers with connecting flights said Peter Ingram, the chief executive of Hawaiian Airlines.However, Mr. Ingram was more positive than other executives interviewed about expanding such a system across the country. “I wouldn’t say that it can’t possibly work,” he said. One of his primary concerns is how to deal with acceptable exceptions to a vaccine mandate.“Who is going to be the adjudicator?” he asked.Is an airplane really as low risk as an operating room?“It’s safe on board, safer than an operating room,” said Deborah Flint, the chief executive of Greater Toronto Airports Authority, during one of the air summit’s panels. This line comes from a widely recognized study, conducted by the Defense Department and published last October, which found that air circulation systems in two types of large Boeing aircraft were even better at filtering out airborne particles than the filtration systems recommended for most hospital operating rooms.Other research has also reinforced that air in a typical cabin is refreshed every two to three minutes, inhibiting the transmission of the coronavirus. But the risk of transmission increases when passengers fail to properly wear masks, Dr. Lin H. Chen, the former president of the International Society of Travel Medicine noted..(A case study published last week reinforced this point: One person not wearing a mask on a two-hour domestic flight in March 2020 in Japan, seems to have infected at least 14 other people despite a modern air filtration system.)Even with a federal mask mandate, some people while on board do and will refuse to wear masks properly. Others cannot wear them for medical reasons, because they are too young or because they are eating or drinking. But even assuming everyone age 2 and above — airlines’ standard age for mask requirements — is properly wearing one, experts still don’t know how new variants are changing the equation, said Dr. David Freedman, a professor of infectious diseases at the University of Alabama at Birmingham. “Delta is much more transmissible,” he said.This is the primary reason behind Canada’s decision to require proof of vaccination for anyone traveling by plane or train starting Oct. 30. “New variants have created new risks for all activities, including travel,” said Sara Johnston, a spokeswoman for Transport Canada, the government agency responsible for operating Canada’s airports and other transportation facilities. Also airports and ramps onto planes don’t typically have the same type of air circulation, Dr. Freedman said.And though vaccinated people are far less likely to get sick, they can still become infected and transmit the virus to another person.Ultimately, the risk in the sky is not the point, some scientists and economists said; it’s the role air travel plays in spreading coronavirus from one part of the country or world to another. The first case of what would later be known as the novel coronavirus was reported to the World Health Organization in China on Dec. 31, 2019. According to a paper in the Journal of Sustainable Tourism, in mid-March, less than three months later, global air transport had carried the virus to 146 countries and reached all continents.Follow New York Times Travel on Instagram, Twitter and Facebook. And sign up for our weekly Travel Dispatch newsletter to receive expert tips on traveling smarter and inspiration for your next vacation. Dreaming up a future getaway or just armchair traveling? Check out our 52 Places list.

A scientific paper expands on social media reports of sudden onset of periods, spotting and other menstrual peculiarities during last summer’s protests in Portland, Ore.At some point last summer, there were just too many reports of protesters who had experienced abnormal menstrual cycles after being exposed to tear gas for Britta Torgrimson-Ojerio, a nurse researcher at the Kaiser Permanente Center for Health Research in Portland, to dismiss them as coincidence.A preschool teacher told Oregon Public Broadasting that if she inhaled a significant amount of gas at night, she’d get her period the next morning. Other Portland residents shared stories of periods that lasted for weeks and of unusual spotting. Transgender men described sudden periods that defied hormones that had kept menstruation at bay for months or years.Dr. Torgrimson-Ojerio decided she would try to figure out whether these anecdotes were outliers or representative of a more common phenomenon. She surveyed around 2,200 adults who said they had been exposed to tear gas in Portland last summer. In a study published this week in the journal BMC Public Health, she reported that 899 of them — more than 54 percent of the respondents who potentially menstruate — said they had experienced abnormal menstrual cycles.“Even though we cannot say anything scientifically definitive about these chemical agents and a causal relationship to menstrual irregularities,” Dr. Torgrimson-Ojerio said, “we can definitively say that in our study most people who had menstrual cycles or a uterus reported menstrual irregularities after reporting exposure to tear gas.”Downstream effects, like the impact on fertility, are not known, but “this is our call to action to ask our scientific community to turn their eye to this issue,” she said.Dr. Torgrimson-Ojerio was also interested in whether people had experienced other problems more than a few hours after being exposed to tear gas. She found that 80 percent of survey participants had, with difficulty breathing being among the most prevalent complaints.Kira Taylor, a professor of epidemiology and population health at the University of Louisville School of Public Health and Information Sciences who is conducting a similar study, said that Dr. Torgrimson-Ojerio’s study provided “some of the first solid evidence” that tear gas might be linked to menstrual abnormalities. It is also “the first study to document the longer-term effects of tear gas exposure in a large population,” she said.Sven-Eric Jordt, a professor of anesthesiology, pharmacology and cancer biology at the Duke University School of Medicine, who was not involved in the study, applauded the work.A tear gas canister striking a barrier in Portland in July.Mason Trinca for The New York TimesMost of the research that police agencies and the government rely on to inform them about tear gas safety “are outdated, often 50 to 70 years old, and don’t measure up to modern toxicological approaches,” he said. “Most of these studies were conducted in young healthy men at the time, either police or military, and not in women, or in a general civilian population representing protesters.”Dr. Torgrimson-Ojerio and her colleagues recruited survey participants through social media and links on the websites of The Oregonian and the Oregon Health Authority in July and August.The researchers asked participants to explain precisely how their periods had been affected after exposure to tear gas. Increased cramps, unusual spotting and uncharacteristically intense or long bleeding were the most common reactions. A number of people who don’t usually have periods because of hormone therapy or age reported unexpected bleeding and spotting, Dr. Torgrimson-Ojerio said.This study has limitations. It is not a random sample.“It is possible that people who feel that their health was damaged by tear gas might have been more likely to respond than people who were also exposed, yet did not feel such harmful effects,” Dr. Taylor said. “This means that some of the numbers might be exaggerated.”Given that subjects were permitted to participate anonymously, researchers could not verify their accounts.A spent canister of CS gas that was fired during a protest at the Immigration and Customs Enforcement building in Portland in January.Alisha Jucevic for The New York TimesNor can the study answer how or why tear gas might be contributing to menstrual irregularities or to what extent other factors are also involved. The authors acknowledge that the high levels of stress and anxiety among protesters, for example, could also have contributed to the physical response.“It is possible that pain, stress, dehydration and exertion play a role,” Dr. Jordt said. Alternatively, tear gas may act as an “endocrine disrupter,” interfering with normal hormonal function.“The tear gas agent CS, sometimes used by police, is a chlorinated chemical compound and produces additional chlorinated byproducts when burned in the canisters used by the police,” he said. “Exposure to chlorinated chemicals can affect menstrual health.”Alexander Samuel, a molecular biologist in France, has been investigating similar questions since French protesters began reporting menstrual irregularities.He mentioned two additional areas for exploration: whether tear gas is metabolized into cyanide, which may cause heavy menstrual bleeding, and the role a traumatic event may play in altering menstrual cycles.Suspicions about tear gas and menstruation first came up more than a decade ago, during the Arab Spring protests, Dr. Jordt noted.In 2011, Chile also banned the use of tear gas after a study suggested that CS gas could cause miscarriages and harm young children. Three days later, the Chilean police lifted the ban, insisting that the type of tear gas they used was perfectly safe.

A variety of digital and old-fashioned approaches are being used around the globe to confirm Covid-19 vaccination. But the world is still far from a universal standard of proof.It is the latest status symbol. Flash it at the people, and you can get access to concerts, sports arenas or long-forbidden restaurant tables. Some day, it may even help you cross a border without having to quarantine.The new platinum card of the Covid age is the vaccine certificate. It is a document that has existed for more than two centuries, but it has rarely promised to hold so much power over culture and commerce. Many versions of these certificates now come with a digital twist.“It’s been a long time since we’ve had a pandemic that has impacted every facet of society so thoroughly, and then a vaccine,” said Carmel Shachar, the executive director of the Petrie-Flom Center for Health Law Policy, Biotechnology and Bioethics at Harvard Law School. “There is no precedent since 1918, and we definitely didn’t have smartphones in 1918.”Ramesh Raskar, a professor at M.I.T. Media Lab, has been leading an effort to develop a solution that includes both a paper certificate that anyone can easily carry as well as a free digital pass that works even without cell service.We are going to emerge from the pandemic with a new “currency for health,” he said.Figuring out how these passes should be used and what they should look like is dividing lawmakers, business leaders, ethicists, designers and health officials.Paper certificates from Shanghai, São Paulo and Uttar Pradesh, India. Anil Kumar Shukla and Ernesto Londoño/The New York Times An Easy Pass Made of Flimsy PaperA Covid-19 vaccination card does not always convey the significance of the document. Nor are the cards typically designed to counter fraud. Most of the hundreds of millions of people worldwide who got at least one shot of a Covid-19 vaccine in the past few months received a flimsy piece of paper.Unlike the easily recognizable “yellow card” that international travelers have long used to document other kinds of immunizations, the designs for Covid-19 vaccination certificates vary from state to state and country to country. In São Paulo, the cards have a green border. In Shanghai, they are stamped in red. In parts of Mexico and Lebanon, they are the size of passports, with the handwriting of the person who filled it out.In India, the certificate is a fully typed printed page. Next to the pronouncement that reads “Together, India will defeat Covid-19” is a photo of the country’s prime minister, Narendra Modi, a rare flourish.In some parts of the world, vaccine proof has gotten people a range of goods: free popcorn and ice cream, and even discounted beers. But for the most part, they have just allowed people to post selfies or to reassure their acquaintance. Some governments are looking for more formal systems that work on phones and counter fraud. Here is a look at some early efforts.DenmarkA template of Denmark’s vaccine passport.Denmark Ministry of HealthName: Coronapas (Corona Passport)Could it get you an indoor table? Yes. In Denmark, restaurants have been open for just takeout since December and have reopened this month with the caveat that only those with a Coronapas can sit indoors, the ministry of health said.How about a concert or sports game? That, too. As part of the country’s reopening plans, the government sanctioned indoor seating for sports and other stadium events for pass holders.Anything else? On April 6, hair salons, tattoo businesses, massage parlors and driving schools opened exclusively to customers with Coronapas. The Danish government does not track each time the pass is used, the ministry said, but about 3.5 million people had visited the app or site in its first week. The app is also intended to be a way for travelers to show other countries that they have been vaccinated.How do you get one? You have to be fully vaccinated and have tested negative for the coronavirus within the past 72 hours or overcame an infection within the past 180 days. Danish citizens can download the app on their smartphone or visit a website to print the Coronapas, which exudes the somber vibe of an old-school train ticket.Isn’t the European Union also developing a system? Yes. On June 21, the E.U. is expected to introduce a certificate called a Digital Green Pass, with the aim of allowing people who have been vaccinated against the coronavirus to travel more freely. Under the proposed rules, each nation within the bloc could decide which travel restrictions, such as obligatory quarantine, to waive for Digital Green holders. But many countries, including Denmark, say they cannot afford to wait for the Digital Green Pass and are developing their own versions.IsraelIsrael’s Green Pass app features a gif of a family headed on a trip.Name of card: The Green PassCould it get you an indoor table? Yes.How about entry to a concert or sports game? That, too.Anything else? The pass allows you to enter many businesses, including swimming pools, gyms, theaters and wedding halls, as well as cultural events, such as concerts, sports games and religious gatherings. Having the pass may also mean that you may not have to quarantine for 10 to 14 days after international travel.How does it work? In late February, Israel’s ministry of health began offering the Green Pass to fully vaccinated residents and individuals who have recovered from Covid-19. When booking a table at a restaurant, many of the businesses began to ask, “Do you have a Green Pass?” Israelis can print their certificates containing a QR code, download the code onto their phones or flash the app itself.What’s with that family? The app and other Green Pass materials feature an animated illustration of a family of three. The man is wearing shorts, a backpack and a camera around his neck, suggesting he’s on vacation. His son and wife are wearing masks, but their postures are relaxed as they pull their suitcases.Aparna Nair, a professor of science history at the University of Oklahoma who maintains a collection of vaccination certificates going back to the 1820s, said that this detail was noteworthy: “They are using the design of the vaccine passport to form visual connections with life after the pandemic, essentially, the vaccine as a literal passport to the rest of the world.”Are there concerns about requiring it so widely? Absolutely. Seema Mohapatra, a law professor specializing in health care and bioethics, noted that many people in the West Bank and Gaza strip had not been vaccinated, raising equity concerns.EstoniaA template of Estonia’s vaccine app.GuardtimeName of card: VaccineGuardCould it get you an indoor table? Not yet.How about entry to a concert or sports game? Not that either. Some business owners and cultural event organizers have expressed the hope that the Estonian government will waive capacity limitations and other restrictions if they agree to check for the certificate at the door, said Dr. Ain Aaviksoo, the chief medical officer at Guardtime, the company that worked with the Estonian government and the W.H.O. to design the certificate. But this hasn’t happened yet.So what is it good for? As of April 30, it will be most useful for going to and from Finland and back on the ferry.Is that really such a high-demand journey? Yes. About 10 percent of the Estonian working population is employed in Finland, Dr. Aaviksoo said. For years, it has been common for Estonians to stay in Helsinki during the week and then take the two-hour ferry ride home to Tallinn for the weekend. That has not been possible during the pandemic. Finland also fuels Estonia’s summer tourism.How does it work? Estonia says it cannot afford to wait for the E.U.’s Digital Green Pass. Starting April 30, residents can download a vaccination certificate, containing a QR code.New YorkA template of New York State’s Excelsior Pass which shows proof of vaccineNew York StateName: Excelsior PassCould it get you an indoor table? Yes.How about entry to a concert or sports game? That, too.Anything else? The state has been requiring some government employees to use it. It’s up to businesses and private organizations to decide whether they want to require the pass for entry. So far, the system has been used at restaurants, weddings, Madison Square Garden, Barclays Center and Yankee Stadium, according to Eric Piscini, vice president of Emerging Business Networks at IBM Watson Health, which designed it.How does it work? New York began offering downloads of the app in March. It verifies whether someone is fully vaccinated or has recently tested negative. Around 400,000 people throughout the state have downloaded it so far, Mr. Piscini said. The pass generates a QR code that can be scanned to produce a green checkmark or a red X. Those without compatible phones can print out their codes, he said.Who scans green? A person could obtain green not only by being fully vaccinated more than 14 days ago, but also by getting a negative PCR test within three days or a negative antigen test — often referred to as a rapid Covid test — within the past six hours. One challenge: only people vaccinated or tested in New York State can use it. “If you live in New Jersey, you will have to get tested in New York to get added to the database and go to the stadium,” Mr. Piscini said. IBM is trying to figure out how to access data from other states. Some venues may also accept other forms of certification.Will there ever be a national app for the U.S.? No. All of the 138 million Americans who have completed their first shot should have received the same Covid-19 vaccine record card created by the Centers for Disease Control and Prevention. But it’s up to states, universities and businesses to decide whether they want to require these cards or offer a supplemental app. The White House said in March there were no plans for a universal federal vaccination database or a mandate for a single credential. While New York has promoted Excelsior as the way to safely and quickly reopen the state, lawmakers in at least a half-dozen states, including Texas, Florida and Arkansas, have moved to ban businesses from requiring vaccination, saying it is a privacy violation or will slow down reopening of commerce.What differentiates Excelsior visually? It is the only Covid-19 vaccine pass to produce a large illustration of the Statue of Liberty when it is scanned.Isabel Kershner contributed reporting from Jerusalem, Suhasini Raj from New Delhi and Ernesto Londoño from Rio de Janeiro

Multiple studies show that there are antibodies in a vaccinated mother’s milk. This has led some women to try to restart breastfeeding and others to share milk with friends’ children.As soon as Courtney Lynn Koltes returned home from her first Covid-19 vaccine appointment, she pulled out a breast pump. She had quit breastfeeding her daughter about two months earlier because of a medication conflict. But she was off those pills, and she had recently stumbled across research suggesting that antibodies from a vaccinated mother could be passed to her baby through milk.Getting the milk flowing again — a process known as relactation — would not be easy. She planned to pump on every odd-numbered hour from 7 a.m. to 11 p.m. But Ms. Koltes and her husband were eager to finally introduce their 4-month-old daughter to family members, and with children not yet eligible for vaccination, she was willing to try.“I am starting to see very slow progress, so it is all worth it if it means I can protect her,” Ms. Koltes, who lives in Orange County, Calif., said last week — nine days after receiving her first dose of the Pfizer-BioNTech vaccine.Partly because it’s so physically taxing, relactation is not common. (Medication is often also involved.) But over the past few weeks, online forums focused on relactation have been swarmed with newly vaccinated mothers like Ms. Koltes. Some had stopped breastfeeding their children more than a year earlier.“I’m glad I’m not the only one here trying to relactate for this reason!” one woman wrote in a lively thread in a private Facebook group.“Go team vaccine!” another wrote.In stark contrast, other parenting and breastfeeding forums have been simmering with worries that breast milk from a newly vaccinated mother could be dangerous. It’s not only vaccine skeptics who have been encouraging those fears, which researchers say are unfounded: Some pediatricians and vaccine administrators have been urging nursing mothers to dump their milk after they are vaccinated.So which is it? Is breast milk from a vaccinated person a sort of elixir capable of staving off Covid? And if so, are the newly vaccinated mothers sneaking breast milk into older children’s cereal or sharing their extra milk with friends’ babies onto something? Or should nursing mothers hold off on getting vaccinated?The answer, six researchers agreed, is that newly vaccinated mothers are right to feel as if they have a new superpower. Multiple studies show that their antibodies generated after vaccination can, indeed, be passed through breast milk. As with so much to do with the coronavirus, more research would be beneficial. But there is no concrete reason for new mothers to hold off on getting vaccinated or to dump out their breast milk, they said.Does ‘vaccinated breast milk’ contain antibodies?Yes, study after study shows it does contain antibodies. How exactly these antibodies protect the infant from Covid is not yet clear.Rebecca Powell, left, and her research team have collected breast milk samples for analysis at their Mount Sinai hospital lab.James Estrin/The New York TimesIn the first nine months of the pandemic, around 116 million babies were born worldwide, according to Unicef estimates. This left researchers scrambling to answer a critical question: Could the virus be transmitted through breast milk? Some people assumed it could. But as several groups of researchers tested the milk, they found no traces of virus, only antibodies — suggesting that drinking the milk could protect babies from infection.The next big question for breast milk researchers was whether the protective benefits of a Covid vaccine could be similarly passed to babies. None of the vaccine trials included pregnant or breastfeeding women, so researchers had to find lactating women who qualified for the first vaccine rollout.Through a Facebook group, Rebecca Powell, a human milk immunologist at the Icahn School of Medicine at Mount Sinai in Manhattan, found hundreds of doctors and nurses willing to periodically share their breast milk. In her most recent study, which has not been formally published, she analyzed the milk of six women who had received the Pfizer-BioNTech vaccine and four who had received the Moderna vaccine, 14 days after the women had received their second shots. She found significant numbers of one particular antibody, called IgG, in all of them. Other researchers have had similar results.“There is reason to be excited,” said Dr. Kathryn Gray, a maternal fetal medicine specialist at Brigham and Women’s Hospital in Boston, who has conducted similar studies. “We’d presume that could confer some level of protection.”But how do we know for sure? One way to test this — exposing those babies to the virus — is, of course, unethical. Instead, some researchers have tried to answer the question by studying the antibodies’ properties. Are they neutralizing, meaning they prevent the virus from infecting human cells?In a draft of a small study, one Israeli researcher found that they were. “Breast milk has the capacity to prevent viral dissemination and block the ability of the virus to infect host cells that will result in illness,” Yariv Wine, an applied immunologist at Tel Aviv University, wrote in an email.Research is too premature for vaccinated mothers who are breastfeeding to act as if their babies can’t get infected, however, said Dr. Kirsi Jarvinen-Seppo, the chief of pediatric allergy and immunology at the University of Rochester Medical Center. Dr. Jarvinen-Seppo has been conducting similar studies. “There is no direct evidence that the Covid antibodies in breast milk are protecting the infant — only pieces of evidence suggesting that could be the case,” she said.How long might protection last?As long as the baby is consuming the antibody-containing breast milk.Destiny Burgess’s twins were born prematurely. Ms. Burgess and her husband are back at work in Asheville, N.C. One of their older children is in kindergarten. Two are in day care. All of that makes Ms. Burgess worried for her now 3-month-old babies.When a vaccinated friend offered to share some of her milk with the twins, she accepted.“I feel like I have this newfound superpower,” that friend, Olivia de Soria, said. Along with feeding her own 4-month-old and sneaking a bit of her milk into her 3-year-old’s chocolate milk, Ms. de Soria is now sharing her milk with five other families.“They can’t get the shot, so this is giving me a little peace of mind,” said Ms. Burgess. She does wonder, though, how much “vaccinated milk” would be needed to make a dent.The unsatisfying answer is that it’s not clear. What researchers agree on is that a baby who consumes breast milk all day long is more likely to be protected than one who gets just an occasional drop e. But none scoffed at the idea of giving a bit to older children if it’s not a hassle.They also agree that breast milk’s protective benefits work more like a pill that you must take every day than a shot that lasts a decade. This short-term defense — known as “passive protection” — may only last hours or days from the baby’s last “dose,” Dr. Powell said.“It’s not the same as the baby getting vaccinated,” she added.That means “as soon as you stop feeding that breast milk, there is no protection — period,” said Antti Seppo, another breast milk researcher at the University of Rochester Medical Center. Dr. Seppo also found that it took about two weeks after the first shot for the antibodies to show up in the milk and that they peaked after the second shot.How do we know ‘vaccinated breast milk’ is safe?Researchers say they know enough about how vaccines generally affect breast milk not to be concerned.Multiple researchers involved in research on breast milk and the Covid vaccine offered slight variations of the same opinion. “There is no reason to think there is anything about this vaccine that would cause it to be harmful, and there’s reason to believe it would be beneficial,” said Christina Chambers, co-director of the Center for Better Beginnings at the University of California, San Diego.So why are parenting forums brimming with anecdotes about pediatricians telling mothers to wait to get vaccinated until their baby is older or to dump their milk after vaccination? Mostly because lactating mothers were not included in vaccine trials, so researchers have not been able to concretely study risks.But researchers’ confidence that breast milk from Covid-19-vaccinated mothers is safe comes from what is known broadly about how vaccines work.“Unlike pregnancy, where there are theoretical safety concerns, there really aren’t concerns about lactation and vaccination,” said Dr. Kathryn Gray, a maternal fetal medicine specialist at Brigham and Women’s Hospital in Boston.Both the Moderna and the Pfizer-BioNTech products are mRNA vaccines. “The ingredients in the vaccine are mRNA molecules that have a short lifetime and have no way of making their way into milk,” Dr. Seppo said.So is relactation really worth all the effort?Maybe not, one initially enthusiastic mother decides.Nearly two weeks in, Ms. Koltes of Orange County was managing to pump only a few drops of breast milk each session. An email exchange with her pediatrician reinforced that she could not be sure — even if she got the milk flowing — that allowing unmasked, unvaccinated relatives to hold her daughter was safe. She applauded other women having more success with relactation. But for her, that was it.“It does feel like a weight is lifted,” she said of quitting her rigorous pumping schedule. Now all that’s left to do is wait for an actual vaccine for her daughter, she said. Both Pfizer and Moderna have recently begun testing their vaccines on babies as young as 6 months old.