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A nation in the grip of a raging pandemic got a glimmer of hope Monday with the drug maker Pfizer’s announcement that its COVID-19 vaccine showed early success among a small number of people in its drug trial.

But with so many unknowns about the first batch of coronavirus vaccines still in development, vaccine and infectious disease experts warn that the public should be prepared to stay the course with 2020-style precautions for months to come, and perhaps longer. Masks and social distancing deep into 2021 are still likely, they say.

Pfizer, one of the companies farthest along in its vaccine trials, said its early data on a small group of participants suggest the shots may be 90 percent effective at preventing COVID-19. Scientists still don’t know, however, whether the vaccine will be effective in the population at large and, if so, how long that protection might last.

No matter which vaccines ultimately prove effective, there are also significant hurdles in distributing shots to millions of people in far-flung places.

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“This could be the magic bullet, but we have to see how long the effect will last. Maybe we have to take the magic bullet every six months,” Dr. Karen Tashima, director of clinical trials in the Immunology Center at the Miriam Hospital in Rhode Island, said of the Pfizer development. “Worst-case scenario is it only lasts two months. It’s just very preliminary data.”

Researchers around the world are racing to develop dozens of potential vaccines, including four candidates in large-scale trials that involve thousands of people in the United States. Recent data suggest the vaccines that are farthest along show strong protection against the virus in monkeys and produce an immune response in study volunteers.

Pfizer’s new interim analysis, for example, was based only on 94 confirmed infections in a study that has enrolled nearly 44,000 people in six countries. The data have not been independently reviewed by regulators at the US Food and Drug Administration.

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“It’s encouraging. Ninety percent protection is wonderful for a first-generation vaccine. If it holds, it will be fantastic, but it’s early," said Michael Kinch, director of the Centers for Research Innovation in Biotechnology & Drug Discovery at Washington University in St. Louis.

“What ends up often happening with vaccines is the first vaccines you get are imperfect, and we learn as we go along," he said.

Many are anxiously awaiting more complete data expected to be released later this month by Pfizer, and in the coming weeks by the Cambridge biotech Moderna and AstraZeneca about whether their vaccines are safe and effective.

But even if one or more of them get the green light, and some vaccines start to be released late this year, the nation’s top disease expert, Dr. Anthony Fauci, doesn’t expect more widespread distribution until the second or third quarter of next year.

Even as the vaccines start to roll out more widely, researchers will still be tracking the people who took part in their large-scale studies to monitor their health. Federal regulators said the companies should follow them for up to two years.

Regulators also expect any COVID-19 vaccine to prevent the disease or decrease its severity in at least 50 percent of people who are vaccinated. Pfizer’s early data suggest it has surpassed that threshold. (By comparison, annual flu shots usually prevent or reduce the risk of illness by 40 percent to 60 percent, while vaccines for measles and smallpox produce lifelong immunity.)

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“When people get the vaccine, they may feel like ‘I am safe now and I don’t have to wear the mask,’ and we have to makes sure we temper that feeling because we don’t know how long that vaccine effect will last," said Tashima, from Miriam Hospital in Rhode Island.

“We are not going to reach people perfectly who are at risk of spreading the disease," she said. "I don’t think we can stop wearing masks for another year. It will take that long to roll out vaccines to the majority of the population.”

Vaccine specialists also point to significant technical and social-justice issues that could hinder distribution of some of the first vaccines, which would affect how widely they’re received and how long social distancing and masks would remain daily routines. For instance, Pfizer’s candidate needs to remain in super-cold storage — roughly minus 103 Fahrenheit — until ready for use, raising concerns about sufficient capacity of ultracold units to safely deliver and store millions of doses around the world.

In addition, vaccines from both Pfizer and Moderna would require patients to receive two shots, 21 or 28 days apart, respectively. And patients would have to stay with the same type of vaccine for both doses, further complicating distribution and uptake.

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Just one of the four vaccines in large late-stage US trials, the one developed by Johnson & Johnson and Beth Israel Deaconess Medical Center in Boston, would offer protection from COVID-19 after one shot. It does not require super-cold storage.

“The first [vaccine] approved may coincidentally be the best, but one that comes a short time or long time later may be better in terms of safety, efficacy, or ease of distribution,” said Dr. Dan Barouch, director of Beth Israel’s Center for Virology and Vaccine Research.

Yet, Barouch added, “nothing could be worse than there being a safe, effective vaccine but not enough to go around. How many vaccines will be available is unknown.”

At the same time, there is concern that many people may be afraid or hesitant to get a shot, fearing that it may not be safe because the process might have been rushed. That could hinder the race to achieve widescale protection known as herd immunity.

What’s more, longstanding mistrust of the health care system in some communities and cultures, stemming from racist treatment by physicians and researchers, could also undermine the effort.

“A safe and effective vaccine that is not trusted by people will fail," Barouch said.

Then there’s the concern about whether vaccine developers are focused on the right part of the coronavirus. Most of the US vaccines target the virus’s spike protein, which plays a major role in its ability to bind to and infect healthy cells.

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But what if that’s not the best approach?

Kinch, at Washington University, said more vaccine teams need to focus on other parts of the virus to better hedge against the potential that spike proteins aren’t the optimum target. While the Pfizer data suggest that approach may work in people, it’s not clear how durable that protection might be, and whether it works as well in older adults, who are most vulnerable to serious illnesses.

The virus that causes COVID-19 is part of a larger family of coronaviruses that usually cause much milder illnesses, including the common cold. People produce an immune response after catching a cold, but that protection wears off and people are again sickened by the same cold viruses. Kinch said the early Pfizer data don’t indicate whether the same would be true with its vaccine.

“We have to approach it like a 401(k) that uses a balance of stocks and bonds because we want to balance the risks, but instead we are putting our portfolio into the spike protein," Kinch said. “If we hit big, we are golden. But if we miss, we are back to where we were in March."



Kay Lazar can be reached at kay.lazar@globe.com Follow her on Twitter @GlobeKayLazar.