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Understanding what the results from COVID vaccine trials mean for the pandemic.

In November 2020, Pfizer and Moderna shared preliminary results from their coronavirus disease 2019 (COVID-19) vaccine trials. We talked to UNC Health infectious disease expert Cynthia Gay, MD, MPH, who leads the Moderna clinical trial at UNC, about what the data from these trials tell us.

1. Clinical trials show the first two vaccines are about 95 percent effective.

Tests on more than 43,000 people have shown that the Pfizer vaccine is 95 percent effective at preventing symptomatic COVID-19 disease. The Moderna study enrolled more than 30,000 participants in the United States. It showed a vaccine efficacy of 95 percent, as well.

“The data on efficacy, meaning how well the vaccines prevent symptomatic disease, is just so good,” Dr. Gay says. “The data’s also really good on the vaccines being well-tolerated. The efficacy of the vaccine is remarkable and frankly much higher than we were hoping for.”

2. The vaccines appear to be effective in preventing both mild and serious cases of COVID-19.

Of the 170 cases of Covid-19 Pfizer observed in its trial, 162 occurred in the placebo group and eight occurred among the group that received the two-dose vaccine. Of the 10 cases of severe Covid-19 in the Pfizer study, nine were in the placebo group.

In the Moderna study, there were 11 cases of severe disease, all in the placebo group. Also the Moderna trial observed 95 cases of symptomatic Covid-19, five of whom received the vaccine. This important finding suggests the vaccine prevents not only mild cases, but also the type of serious disease that leads patients to die or be hospitalized.

3. Short-term side affects appear to be mild.

The Pfizer study tracked data on common side effects in an 8,000-patient portion of the study. The only severe side effects to occur in more than 2 percent of people were fatigue, which occurred in 3.7 percent of patients after the second dose, and headache, which occurred in 2 percent. Older adults had fewer and milder side effects than younger participants. The Pfizer study has followed 19,000 participants for at least two months since their second dose of the vaccine.

In the Moderna trial, the most common side effects of the vaccine have been some tenderness, swelling, some soreness at the injection site. Other symptoms include feeling fatigued or a mild headache.

“For the Moderna study alone, we know that approximately 15,000 individuals would have received the vaccine. We have over a median of at least two months of safety follow-up after individuals completed the two doses, so a median follow-up of two months of 15,000 people completing the vaccine series gives us a lot of safety data about how many people have these local reactions,” Dr. Gay says. “Those mild symptoms of feeling achy, tired (and) in some, low-grade fever, are really your immune system telling you that the vaccine is doing exactly what we want it to do.”

Researchers will continue to follow the vaccine study participants to determine how long the vaccines might be effective.

“The reason to follow people for 12 months to two years after receiving the COVID-19 vaccines is because we want to know what the durability of the immune response would be, meaning, is this a similar scenario to influenza where we might have to, for example, give a yearly booster vaccine?” Dr. Gay says.

4. The COVID-19 vaccines require two doses.

Study data showed that two doses of the vaccine needed to be given.

“Earlier data show that the second dose improves the immune responses as opposed to just one,” says Dr. Gay.

It is not unusual for people to require more than one dose to receive maximum protection from a vaccine.

5. COVID-19 vaccine effectiveness in children has not yet been fully studied.

Children, pregnant women, breastfeeding women and those who are immunocompromised (meaning they have a weakened immune system) were not included in phase II or III of the clinical trials for the COVID-19 vaccines, so it is not known yet if the vaccine would be effective for these populations. Further study is needed.

Cynthia Gay, MD, MPH, is an associate professor of medicine in the division of infectious diseases in the UNC School of Medicine. She is a practicing physician and the medical director of the UNC HIV Cure Center.

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