Pfizer and BioNTech said on Wednesday they were now applying to the Food and Drug Administration for supplemental approval of a coronavirus vaccine booster shot for those aged 16 and up, and will submit all their supporting data by the end of this week. The move came as the companies said that a third shot of the vaccine sharply increased the levels of antibodies against the virus.The companies conducted a study of 306 volunteers who received a booster shot about five to eight months after their second shot. Researchers found that the level of antibodies that block the coronavirus jumped more than three times higher than the level after the second dose.The side effects of a third injection were about the same as after the initial two doses, the companies said. The underlying data was not included in the news release, nor were the dates or location of the study specified. The companies said they were preparing a scientific publication describing the research.The news of Pfizer and BioNTech’s booster application came two days after the F.D.A. fully approved their two-dose vaccine for those 16 and older, making it the first to move beyond emergency use status.Over the past few weeks, federal regulators have been racing to collect and evaluate data on booster shots. If the F.D.A. decides additional shots are safe and effective, the Biden administration has said it wants adults to get a third injection eight months after their second shot of the Pfizer or Moderna vaccines, starting the week of Sept. 20.Federal health officials said last week that they believe that the potency of the Pfizer-BioNTech and Moderna vaccines wanes over time, raising the risk of infection from the highly contagious Delta variant. While data indicate that the vaccines continue to offer robust protection against hospitalization and severe disease, the officials said they fear that the situation could change without booster shots.Some public health experts have challenged the plan as premature, saying the available data shows that the vaccines are holding up well against severe disease and hospitalization, including against the Delta variant. Extra shots would be warranted only if the vaccines failed to meet that standard, some have said.Pfizer executives presented an early look at their booster data on July 23, during their second-quarter earnings call. In a smaller study, they found that antibody levels dropped markedly in the months following a second dose. But those levels jumped back up after a third dose. When researchers expanded their focus to a larger group of subjects, they continued to find a strong effect from the boosters.Antibodies that can neutralize the coronavirus are only one kind of defense our immune systems use to fight it. The new study did not include details about other defenses provoked by the vaccine, such as immune cells trained to kill infected cells.The participants in the new booster study were between the ages of 18 and 55. It was not immediately clear why the study did not include older people. Volunteers were followed for a median period of 2.6 months..css-1xzcza9{list-style-type:disc;padding-inline-start:1em;}.css-3btd0c{font-family:nyt-franklin,helvetica,arial,sans-serif;font-size:1rem;line-height:1.375rem;color:#333;margin-bottom:0.78125rem;}@media (min-width:740px){.css-3btd0c{font-size:1.0625rem;line-height:1.5rem;margin-bottom:0.9375rem;}}.css-3btd0c strong{font-weight:600;}.css-3btd0c em{font-style:italic;}.css-w739ur{margin:0 auto 5px;font-family:nyt-franklin,helvetica,arial,sans-serif;font-weight:700;font-size:1.125rem;line-height:1.3125rem;color:#121212;}#NYT_BELOW_MAIN_CONTENT_REGION .css-w739ur{font-family:nyt-cheltenham,georgia,’times new roman’,times,serif;font-weight:700;font-size:1.375rem;line-height:1.625rem;}@media (min-width:740px){#NYT_BELOW_MAIN_CONTENT_REGION .css-w739ur{font-size:1.6875rem;line-height:1.875rem;}}@media (min-width:740px){.css-w739ur{font-size:1.25rem;line-height:1.4375rem;}}.css-9s9ecg{margin-bottom:15px;}.css-16ed7iq{width:100%;display:-webkit-box;display:-webkit-flex;display:-ms-flexbox;display:flex;-webkit-align-items:center;-webkit-box-align:center;-ms-flex-align:center;align-items:center;-webkit-box-pack:center;-webkit-justify-content:center;-ms-flex-pack:center;justify-content:center;padding:10px 0;background-color:white;}.css-pmm6ed{display:-webkit-box;display:-webkit-flex;display:-ms-flexbox;display:flex;-webkit-align-items:center;-webkit-box-align:center;-ms-flex-align:center;align-items:center;}.css-pmm6ed > :not(:first-child){margin-left:5px;}.css-5gimkt{font-family:nyt-franklin,helvetica,arial,sans-serif;font-size:0.8125rem;font-weight:700;-webkit-letter-spacing:0.03em;-moz-letter-spacing:0.03em;-ms-letter-spacing:0.03em;letter-spacing:0.03em;text-transform:uppercase;color:#333;}.css-5gimkt:after{content:’Collapse’;}.css-rdoyk0{-webkit-transition:all 0.5s ease;transition:all 0.5s ease;-webkit-transform:rotate(180deg);-ms-transform:rotate(180deg);transform:rotate(180deg);}.css-eb027h{max-height:5000px;-webkit-transition:max-height 0.5s ease;transition:max-height 0.5s ease;}.css-6mllg9{-webkit-transition:all 0.5s ease;transition:all 0.5s ease;position:relative;opacity:0;}.css-6mllg9:before{content:”;background-image:linear-gradient(180deg,transparent,#ffffff);background-image:-webkit-linear-gradient(270deg,rgba(255,255,255,0),#ffffff);height:80px;width:100%;position:absolute;bottom:0px;pointer-events:none;}.css-uf1ume{display:-webkit-box;display:-webkit-flex;display:-ms-flexbox;display:flex;-webkit-box-pack:justify;-webkit-justify-content:space-between;-ms-flex-pack:justify;justify-content:space-between;}.css-wxi1cx{display:-webkit-box;display:-webkit-flex;display:-ms-flexbox;display:flex;-webkit-flex-direction:column;-ms-flex-direction:column;flex-direction:column;-webkit-align-self:flex-end;-ms-flex-item-align:end;align-self:flex-end;}.css-12vbvwq{background-color:white;border:1px solid #e2e2e2;width:calc(100% – 40px);max-width:600px;margin:1.5rem auto 1.9rem;padding:15px;box-sizing:border-box;}@media (min-width:740px){.css-12vbvwq{padding:20px;width:100%;}}.css-12vbvwq:focus{outline:1px solid #e2e2e2;}#NYT_BELOW_MAIN_CONTENT_REGION .css-12vbvwq{border:none;padding:10px 0 0;border-top:2px solid #121212;}.css-12vbvwq[data-truncated] .css-rdoyk0{-webkit-transform:rotate(0deg);-ms-transform:rotate(0deg);transform:rotate(0deg);}.css-12vbvwq[data-truncated] .css-eb027h{max-height:300px;overflow:hidden;-webkit-transition:none;transition:none;}.css-12vbvwq[data-truncated] .css-5gimkt:after{content:’See more’;}.css-12vbvwq[data-truncated] .css-6mllg9{opacity:1;}.css-qjk116{margin:0 auto;overflow:hidden;}.css-qjk116 strong{font-weight:700;}.css-qjk116 em{font-style:italic;}.css-qjk116 a{color:#326891;-webkit-text-decoration:underline;text-decoration:underline;text-underline-offset:1px;-webkit-text-decoration-thickness:1px;text-decoration-thickness:1px;-webkit-text-decoration-color:#326891;text-decoration-color:#326891;}.css-qjk116 a:visited{color:#326891;-webkit-text-decoration-color:#326891;text-decoration-color:#326891;}.css-qjk116 a:hover{-webkit-text-decoration:none;text-decoration:none;}Pfizer and BioNTech said that in addition to the F.D.A., they plan to submit their data to regulatory authorities in Europe and other countries.The administration’s booster plan does not as yet include recipients of Johnson & Johnson’s one-shot vaccine. Johnson & Johnson announced earlier on Wednesday that unlike the studies of Moderna’s and Pfizer’s vaccines, a study of 17 volunteers showed little change in their antibody levels over the course of six months.But the study also showed that when the volunteers were given a second shot six months after their first, their antibodies against the coronavirus jumped nine times higher than the level after the first dose. Company officials said they are looking forward to discussing a potential booster strategy for their vaccine with federal health officials.While Pfizer-BioNTech vaccine was fully approved as a two-shot regimen for those 16 and older, adolescents aged 12 to 15 can continue to be vaccinated under the vaccine’s emergency use authorization. Regulators have only authorized a third shot for some people with weakened immune systems.