Over the summer, the Food and Drug Administration announced that in order for an experimental Covid-19 vaccine to get the green light, it would need to be safe and “prevent disease or decrease its severity in at least 50 percent of people who are vaccinated.”

In fact, no vaccine is 100 percent effective, but some work better than others. One of the most successful is the measles vaccine — two doses are 97 percent effective in preventing the disease, according to the Centers for Disease Control and Prevention.

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Still, 50 percent sounds like a far cry from 97 percent.

“I know that 50 percent does sound low but that is still some protection, and some protection is better than no protection,” said Dr. Jeff Kwong, professor of public health and family medicine and interim director of the Centre for Vaccine Preventable Diseases at the University of Toronto.

Based on the effectiveness threshold the FDA has set for a Covid-19 vaccine to be approved or granted emergency use authorization, it’s possible a vaccine becomes available that helps only half of people receiving it, while offering no benefit to the other half. It’s also possible that a vaccine could have different effects in different people – helping to prevent disease in some people while reducing the severity of Covid-19 in others.

But even if a Covid-19 vaccine is only 50 percent effective, it could make a difference, experts say, pointing out that the flu vaccine effectiveness can vary widely from year to year, anywhere from 20 percent to 60 percent over the last decade, and the shots still offer benefits. When the flu vaccine matches up well with the influenza viruses that are circulating, the shots can reduce illness, hospitalizations and deaths, the CDC says.

And if enough people were immunized, a Covid-19 vaccine wouldn’t need to be near-perfect to have a substantial impact in the community, said Dr. Gregory Poland, director of the Mayo Clinic’s Vaccine Research Group and editor-in-chief of the journal Vaccine.

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“If you had a 60 or 70 percent effective vaccine and everybody took it, you might actually be reaching toward herd immunity and potentially then dampen down this pandemic,” Poland said.

But researchers won’t know how effective the experimental vaccines are — if they are effective at all — until studies are completed.

Related: “I think that it could really benefit the world, and I think it could also help scientists know more about the coronavirus,” said a 12-year-old who recently received his first injection.

“Right now, we have no idea if any of these vaccines work,” said vaccine researcher Dr. Peter Hotez, dean of the National School of Tropical Medicine at the Baylor College of Medicine in Houston.

Most clinical trials underway now primarily are assessing whether Covid-19 vaccines prevent disease, Poland explained. Study results also may include additional information, such as whether vaccinated people who still contracted the virus became less sick than unvaccinated people who contracted it.

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Once the studies are complete, they also may reveal who benefited the most from vaccination. Data, for instance, may show how a vaccine affected men as against women, various age groups, or people with underlying health conditions. Future studies and longer follow-ups may provide additional details about whether the vaccines ultimately reduce hospitalizations and deaths.

Availability of a vaccine likely won’t mean an immediate end to the pandemic, experts say.

Because it would take time to immunize millions of people, precautions such as masks, social distancing and hand-washing would remain important as long as the virus is circulating in the community.

Even if the first generation of Covid-19 vaccines aren’t as effective as hoped, Hotez said he’s not planning to pass up any of them. There might be the possibility he could get a booster later or even receive a different, more effective vaccine at some point, he said. For now, though, he would take what he can get.

“I would feel much better knowing that I got vaccinated and have some levels of virus-neutralizing antibody in my system,” he said. “Even if it’s not the best vaccine, it still could prevent me from going to the hospital or worse.”

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