Both the National Academy of Medicine and the C.D.C. have urged that older people be among the first groups to receive vaccines. It’s possible that vaccines under development by Novavax and Sanofi, which are likely to begin late-phase clinical trials later this year, may be better for the elderly, Dr. Offit noted. Those vaccines contain immune-stimulating particles like the ones contained in the Shingrix vaccine, which is highly effective in protecting older people against shingles disease.

Can it be rolled out effectively? The Pfizer vaccine, unlike others in late-stage testing, must be kept supercooled, on dry ice around 100 degrees Fahrenheit below zero, from the time it is produced until a few days before it is injected. mRNA quickly self-destructs at higher temperatures. Pfizer has devised an elaborate system to transport the vaccine by truck and specially designed cases to vaccination sites. Public health workers are being trained to handle the vaccine as we speak, but we don’t know for sure how well it will do if containers are left out in the Arizona sun too long. Mishandling the vaccine along the way from factory to patient would render it ineffective, so people who received it could think they were protected when they were not, Dr. Offit said.

Could a premature announcement hurt future vaccines? There’s no way at present to know whether the Pfizer vaccine will be the best over all or for specific age groups. But if the F.D.A. approves it quickly, that could make it harder for manufacturers of other vaccines to carry out their studies. If people are aware that an effective vaccine exists, they may decline to enter clinical trials, partly out of concern they could get a placebo and remain unprotected. Indeed, it may be unethical to use a placebo in such trials. Many vaccines will be needed in order to meet global demand for protection against Covid-19, so it’s crucial to continue additional studies.

Could the Pfizer study expedite future vaccines? Scientists are vitally interested in whether the small number who received the real vaccine but still got sick produced lower levels of antibodies than the vaccinated individuals who remained well. Blood studies of those people would help scientists learn whether there is a “correlate of protection” for Covid-19 — a level of antibodies that can predict whether someone is protected from the disease. If they had that knowledge, public health officials could determine whether other vaccines under production were effective without necessarily having to test them on tens of thousands of people.

But it’s difficult to build such road maps. Scientists have never established correlates of immunity for pertussis, for example, although vaccines have been used against those bacteria for nearly a century.

Still, this is good news, said Dr. Joshua Sharfstein, a vice dean at the Johns Hopkins Bloomberg School of Public Health and a former F.D.A. deputy commissioner. He said: “I hope this makes people realize that we’re not stuck in this situation forever. There’s hope coming, whether it’s this vaccine or another.”

Arthur Allen is a reporter for Kaiser Health News and the author of “Vaccine: The Controversial Story of Medicine’s Greatest Lifesaver.” This essay was copublished with Kaiser Health News.

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