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Surprisingly strong US clinical trial results reported by AstraZeneca on Monday raised the prospect of a fourth COVID-19 vaccine in the United States soon — and a potential supply surplus just as the general public becomes eligible for shots in Massachusetts and most other states next month.

“This could have a huge impact,” said Dr. Daniel Kuritzkes, chief of infectious diseases at Brigham and Women’s Hospital. “When it comes to COVID vaccines, you can’t have too many of them.”

But there was also caution around AstraZeneca’s vaccine. About a dozen countries temporarily paused using it last week due to safety concerns, though the concerns were later dismissed by European regulators and didn’t surface in the US study. Even so, some worried the pause could heighten the broader vaccine skepticism that public health authorities have been fighting.

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AstraZeneca said Monday that its two-shot vaccine candidate, already authorized in scores of countries, showed a 79 percent efficacy rate in stopping symptomatic COVID-19 across all adult age groups in the large-scale US study. More crucially, it showed a 100 percent rate in preventing severe illness and hospitalization.

“This vaccine can play an important role in protecting millions of people worldwide against this lethal virus,” said Mene Pangalos, an AstraZeneca executive vice president for biopharma research and development. He said the company is preparing to submit its findings to American regulators and seek emergency use authorization.

The announcement, released overnight from AstraZeneca’s headquarters in the United Kingdom, injected a wild card into the US vaccination drive. Biden administration officials last week promised governors that vaccine supplies, largely stagnant for weeks, would begin to surge by the end of March as more doses of the three currently authorized vaccines — from Pfizer-BioNTech, Moderna, and Johnson & Johnson — come off their production lines.

Adding a fourth vaccine to the mix could be a windfall. But federal officials were circumspect, noting the Food and Drug Administration staff and a group of independent medical advisers will have to study the full data from the AstraZeneca study, which have yet to be published, before the FDA green-lights the vaccine.

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Last May, the Trump administration purchased 300 million vaccine doses from AstraZeneca. Millions of those doses have already been produced and stockpiled in the United States, but it wasn’t clear Monday how soon the vaccine could join the US vaccine pipeline or what role it will play. The other three vaccines got the FDA’s nod within weeks after announcing their clinical results.

Adding to the uncertainty, White House officials on Thursday said they were finalizing a plan to loan 4 million of the AstraZeneca doses stored here to Canada and Mexico, with the understanding those countries would later send doses here.

“It’s too early for us to declare that we’re in a surplus position,” Andy Slavitt, the White House senior adviser for its COVID-19 response, said at a press briefing Monday. “Obviously, the more we get confidence in our increased supply, the more flexibility that gives us for different events down the road, whether it’s vaccinating adolescents, whether it is sharing that supply with other countries.”

White House officials said about 44 million Americans, roughly 13 percent of the population, already have been fully vaccinated.

AstraZeneca’s late-stage clinical trial was conducted mostly in the United States, along with some trial sites in Chile and Peru. The company tested 32,449 volunteers; two-thirds were given the vaccine and one third a dummy dose known as a placebo. All were given two doses, four weeks apart. An independent monitoring board found no safety concerns, such as the blood clots that prompted countries in Europe and elsewhere to temporarily suspend use of the vaccine.

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Some industry analysts had soured on the AstraZeneca vaccine, concerned that the blood clot fears, even if unwarranted, could undermine confidence. But the US data seemed to give it fresh momentum. The vaccine’s 79 percent efficacy rate surpassed the 62 percent rate seen in its trial in the United Kingdom last year.

In a research note, Peter Welford, a health care analyst for the investment bank Jefferies, called the findings “surprisingly positive.”

The vaccine, developed by Oxford University researchers, uses a harmless cold virus to prime the immune system to fend off the coronavirus. It was initially considered a global leader in the race to field a vaccine against COVID-19. But the company stumbled in sorting out the appropriate vaccine dose and the interval between doses in European trials, which delayed its US clinical study and allowed rival vaccines to take the lead.

Even so, AstraZeneca’s vaccine had become a backbone of Europe’s vaccine program until last week, when France, Germany, Italy, and other countries suspended its use, against the advice of regulators, after reports that it was linked to blood clots.

The vaccine was returned to use in those countries later in the week after European Medicines Agency investigators concluded the vaccine carried no increased risk of blood clotting.

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Medical experts said the AstraZeneca vaccine appears safe.

“There was no evidence of a cause-and-effect relation, and no evidence of an increased risk of blood clots,” said Dr. Philip Landrigan, director of Boston College’s global public health program. “So we’re in a happy position to have a fourth vaccine in a matter of weeks. It’s clearly a very good, very effective vaccine.”

But with vaccine skepticism already a problem on both sides of the Atlantic, as health officials struggle to subdue the virus and a growing number of variants, doctors worry that the blood clot scare could add to a toxic mix of anti-vaccination fear and misinformation.

“There is certainly concern that anything that even temporarily casts some doubt about the safety of the vaccine can fuel hesitancy,” said Kuritzkes, at Brigham and Women’s. “It’s concerning to those of us in public health if people are hesitant, because it’s not justified.”

In the White House briefing, Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases and Biden’s chief medical adviser, said the US study found the virus effective for all of the groups studied, including people over 65 and those with chronic health conditions. He emphasized there were “no specific safety concerns related to the vaccine,” including no evidence of blood clotting.

“There are very many countries in Europe and throughout the world who have already authorized this,” Fauci said. “So the fact that a US-run study has confirmed the efficacy and the safety of this vaccine I think is an important contribution to global health in general.”

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Although President Biden has said that there should be enough vaccines nationally to inoculate every adult who wants them by the end of May, Governor Charlie Baker has said that supply continues to be tight in Massachusetts. He said that a dramatic ramping up of the federal vaccine supply will be critical on April 19, when all state residents over 16 become eligible for a shot.

About 800,000 more Massachusetts residents qualified for shots Monday, including people ages 60 to 64 and some categories of essential workers. Baker last week said his goal is to vaccinate more than 4 million adults by July 4. The number of state residents fully vaccinated topped 1 million Friday.





Robert Weisman can be reached at robert.weisman@globe.com. Follow him on Twitter @GlobeRobW.