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Early Trial Data Show Pfizer, Moderna Coronavirus Vaccines are Highly Effective

Whether the vaccine will work just as well in older adults is yet to be determined

older man getting a vaccination

Halfpoint Images/Getty Images

En español | Just one week after Pfizer announced that its coronavirus vaccine candidate was found to be more than 90 percent effective at preventing COVID-19 in preliminary clinical trial results, biotech firm Moderna says it, too, may have a highly effective vaccine. 

Early data from Moderna’s phase 3 clinical trial show its vaccine is nearly 95 percent effective at preventing COVID-19, including severe cases of the illness caused by the new coronavirus. The announcement was made in a news release by the National Institute of Allergy and Infectious Diseases, part of the National Institutes of Health, which codeveloped the vaccine. 


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The promising news from both drugmakers comes as coronavirus cases surge to record numbers in the U.S. On Nov. 12, the Centers for Disease Control and Prevention (CDC) recorded more than 194,000 new cases of COVID-19 — the highest since the start of the pandemic. Hospitalizations are also at all-time highs, and the death toll has surpassed 246,200.  

Both Moderna and Pfizer are expected to release more detailed findings from their late-stage clinical trials in the next few weeks. Pfizer has said that its trial will continue until 164 trial participants develop COVID-19. (The interim analysis evaluated 94 confirmed cases.) The company will also study how well the vaccine does at preventing severe disease and preventing reinfection among those who have already had COVID-19. If Pfizer’s phase 3 trial meets the required safety milestones, the company will submit its vaccine to the U.S. Food and Drug Administration (FDA) for approval through emergency-use authorization (EUA).

Moderna announced that it also intends to submit for an EUA in the coming weeks and expects the EUA to be based on the final analysis of 151 COVID-19 cases (95 were included in the interim analysis) and a median follow-up of more than two months. Even still, experts predict it will be several months before a coronavirus vaccine is widely available to the public.

“I think it is a good thing,” William Schaffner, M.D., an infectious disease specialist and professor of preventive medicine at the Vanderbilt University School of Medicine, said about Pfizer’s news, announced Nov. 9. Still, he cautions that there’s a lot more to learn, especially when it comes to how well the vaccines will work in older adults, who so far account for 95 percent of COVID-19 deaths in the U.S.


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Older people, in general, respond differently to vaccines than younger adults “because their immune systems aren’t as robust,” Schaffner explains. And data from older adults and other subpopulations who have been disproportionately affected by the coronavirus have yet to emerge. “So we look forward to seeing those results,” he says. “And we hope that this high level of effectiveness — 90 percent or higher — is also maintained in the older population.”

Both Pfizer and Moderna’s vaccines require two doses and use a similar mRNA technology, which introduces a genetic code the body can use to make its own viral protein to induce an immune response. They are two of four vaccine candidates in late-stage clinical trials in the U.S.

Pfizer has enrolled 43,538 participants in its study; 42 percent of global participants are from “racially and ethnically diverse backgrounds,” the company reports, as are 30 percent of U.S. participants. Moderna has more than 30,000 adult participants at 100 clinical research sites in the U.S.; 37 percent of trial volunteers are from racial and ethnic minorities. Moderna’s phase 3 study also includes more than 7,000 Americans over the age of 65 and roughly 5,000 volunteers who are under the age of 65 but have a chronic disease that puts them at increased risk of severe COVID-19.

Earlier this year the FDA told vaccine makers that it will issue an EUA only for candidates that are at least 50 percent effective at preventing a coronavirus infection. For comparison, the influenza vaccine is between 40 and 60 percent effective at preventing the flu or reducing the severity of the illness. The measles vaccine is about 93 percent effective.

“With today’s news, we are a significant step closer to providing people around the world with a much-needed breakthrough to help bring an end to this global health crisis,” Pfizer Chairman and CEO Albert Bourla said in a statement. “We look forward to sharing additional efficacy and safety data generated from thousands of participants in the coming weeks.”

Both companies released their interim findings in a press release, not in a peer-reviewed journal.

Long-term safety, protection won't be known for some time

Although short-term efficacy and safety concerns will come to light in the clinical trials, the vaccine’s long-term safety won’t be known anytime soon, Schaffner points out. But “many vaccine safety monitoring systems watch for adverse events” after a vaccine is authorized or approved, according to the CDC. If unexpected side effects and adverse events occur, they are evaluated and, if necessary, vaccine recommendations are adjusted.

Another question that won’t be answered right away: how long the vaccine will provide protection against a coronavirus infection. Some vaccines work for years, but others require more frequent follow-up.

“We’re so interested in trying to reduce the impact of this COVID pandemic on our population — after all, 1,000 people or more are dying every day [in the U.S.] — we can’t have all the answers right at the beginning,” Schaffner says.

If either vaccine receives the green light from the FDA, a panel of experts will meet to determine who should receive the medicine first. Pfizer predicts it can produce up to 50 million vaccine doses in 2020 and up to 1.3 billion doses in 2021. Moderna says it will have approximately 20 million doses of its vaccine ready to ship in the U.S. by the end of 2020 and remains on track to manufacture 500 million to 1 billion doses globally in 2021.

More than one vaccine can receive federal authorization or approval, Schaffner notes. And having more than one safe, effective vaccine will increase the likelihood that “we’ll be able to reach out and vaccinate everyone,” he says. “Who knows — we may find out that one vaccine works better in one subset of the population than another. We’ll have to keep our minds open about that.”

What Is an EUA?

  • An emergency-use authorization (EUA) allows the FDA to provide more timely access to drugs and devices during a public health emergency.

  • The EUA process is different from the traditional approval process. For starters, it's much faster. (Standard approval can take six to 10 months.) For an EUA, the FDA evaluates options using the evidence available and balances the risks and benefits.

  • Both the EUA and standard approval pathways require data demonstrating a drug's safety and effectiveness.

  • Issuance of an EUA for a coronavirus vaccine “would require a determination by FDA that the vaccine's benefits outweigh its risks based on data from at least one well-designed phase 3 clinical trial that demonstrates the vaccine's safety and efficacy in a clear and compelling manner,” the agency states in its guidance to vaccine makers.

  • Vaccine makers who apply for an EUA for a coronavirus vaccine are also encouraged to “work toward submission of a Biologics License Application (BLA) as soon as possible,” the FDA says. They must also plan to assess long-term safety and efficacy.

  • Before issuing an EUA for a coronavirus vaccine, the FDA will meet with an advisory committee “to discuss whether the available safety and effectiveness data support authorization of an EUA for the specific request under review."

Editor’s Note: This story, originally published Nov. 9, has been updated to include new information.

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