Moderna Vaccine Is Highly Protective and Prevents Severe Covid-19, Data Show

But fewer cases of Covid-19 occurred among trial participants in the early days of the Moderna trial, making it harder to measure differences between the vaccinated group and the placebo group. In any case, health officials have said that for both vaccines, two doses are essential for full protection.

A second difference involves the ability to prevent severe disease. Moderna produced more evidence that its vaccine can do so, according to the review. In its trial, 30 volunteers developed severe cases of Covid. All of them were in the placebo group, with no cases among the people who were vaccinated.

In the Pfizer-BioNTech trial, the findings were less compelling. There were 10 severe cases in the placebo group and one in the vaccinated group. Those numbers are too small to gauge the vaccine’s ability to prevent serious illness.

“The available data for these outcomes did not allow for firm conclusions,” the F.D.A. said.

The documents published Tuesday made clear that side effects were particularly common after the second dose, but typically lasted only a day. Experts say people may need to take a day off work after getting the shot.

During the Moderna trial, researchers also kept an eye out for volunteers who developed new disorders. In a trial lasting several months with 30,000 volunteers, it is normal for some to come down with conditions that have nothing to do with the vaccine, health experts say. Comparing the rates between the people who get the vaccine and the placebo — as well as general background rates — can help pinpoint serious concerns and rule out coincidences.

During the Moderna trial, three vaccinated participants developed a form of temporary facial paralysis called Bell’s palsy, while one participant with the placebo also experienced it. Bell’s palsy, which can last weeks or longer, can be triggered by viral infections and other causes. Some 40,000 people develop the condition each year in the United States. Years of intense research has failed to find evidence that any vaccine routinely recommended in the United States causes Bell’s palsy.

In the review released on Tuesday, the F.D.A. said: “Currently available information is insufficient to determine a causal relationship with the vaccine.”

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