要不要打第3针疫苗？

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Covid-19 Booster: Do I Need a Third Vaccine Shot?
WSJ Staff
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Last Updated August 6, 2021, 11:34 AM
The Food and Drug Administration is working on a strategy for Covid-19 vaccine boosters that would lay out when and which vaccinated individuals should get the follow-up shots. Here is what we know and don't know about the need for boosters.

Do I need a third dose of a Covid-19 vaccine?

Studies are under way exploring whether people will need to take a third dose of a vaccine -- or a second dose in the case of the Johnson & Johnson single-shot vaccine -- to stay protected against Covid-19.

Physicians and public-health experts say you probably will need a booster shot. Although some vaccines, such as the one for measles, offer lifelong protection against a virus, other shots don't. The vaccines for tetanus and diphtheria require a booster every 10 years, for instance, and women are supposed to get the whooping-cough vaccine each time they are pregnant, according to the Centers for Disease Control and Prevention's vaccine recommendations.

Will Covid-19 immunity last?

Many researchers suspect the immunity provided by Covid-19 vaccines will wane over time. They have that expectation partly because the natural immunity people develop against milder coronaviruses declines. "Historically, at least with the coronaviruses, the mild common cold coronaviruses, the durability of the protection from infection isn't very long," Anthony Fauci, director of the National Institute of Allergy and Infectious Disease, said at The Wall Street Journal's Tech Health conference in June. Another reason researchers say people will probably need a booster shot is for protection in case any coronavirus variants emerge that prove able to evade current vaccines.

The vaccines work by generating neutralizing antibodies, which prevent the virus from entering cells and replicating.

Pfizer has said that antibody levels start to decline from their earlier peaks about eight months after the second dose. After a third shot, antibody levels were up more than five times among people ages 18 to 55 and more than 11 times in people ages 65 to 85, compared with two doses, according to Pfizer's data.

Even with reduced antibodies, vaccines are protecting against severe disease, according to researchers and published research. That is because the vaccines help other immune-system weapons: the T-cells that hunt down infected cells and destroy them, and memory B-cells, which circulate in the blood and help churn out antibodies upon detecting a virus. Both are likely helping prevent severe disease, according to researchers.

What about people with weakened immune systems?

The Biden administration is pushing for the swift release of a booster strategy because some populations -- people age 65 or older and people who are immunocompromised, as well as those who got the shots in December or January shortly after they were rolled out -- could need boosters as soon as this month, people familiar with the FDA's strategy told The Wall Street Journal.

The immunocompromised include transplant recipients, some cancer survivors and people living with HIV; their weakened immune systems make them less responsive to vaccines, which stimulate the immune system to provide protection. The elderly also tend to have weaker immune systems and they are also likely candidates for boosters. France and Israel are already giving a third shot to some at-risk people. However, the World Health Organization has called for a halt on boosters until at least the end of September, citing an urgent need to vaccinate the rest of the world.

How often will I need a booster?

It may be every year, 18 months or even two years. It could be a different time frame. It's unknown. Whether and how often people would need to get a booster shot is under study right now. The Covid-19 vaccines haven't been around long, so not much time has passed to assess how long their protection lasts.

Pfizer said in July that efficacy of the vaccine protecting against symptomatic disease dropped every two months, to 84% after six months from a peak of 96% within two months of vaccination. Researchers can sort out how often people would need to get a booster shot after figuring how long the vaccines provide their fullest protection against Covid-19.

Still, the current two-shot regiment has been holding up well, even against the highly infectious Delta variant, by preventing severe illness in most people.

Is it safe for me to get a booster shot from a different vaccine brand?

That is under study right now. In June, NIAID, which is part of the National Institutes of Health, started a study looking at whether mixing and matching vaccines and boosters could prolong immunity and better protect against variants. One thing the study will explore is the best combination of shots. It may be that getting a vaccine from one manufacturer and a booster from another might actually produce stronger protection, researchers say. The ability to mix and match might also simplify the logistics of giving people boosters.

Will boosters better protect against Delta or other variants?

Researchers say the current vaccines appear to work well against variants of concern that have emerged so far. Yet they may not work as strongly against newer variants as they do against earlier ones. For that reason, people may want booster shots to ensure they get the fullest level of protection possible against the variants. And they would want a third shot if any variants emerge that prove better able to evade current vaccines. Drugmakers are working on shots targeting variants.

How does a booster shot work?

A booster shot would be akin to getting another vaccine dose. The extra dose mobilizes your body's immune system defenses further to protect against Covid-19, especially variants such as Delta that are better at eluding vaccine-generated protection. Pfizer and partner BioNTech said early testing indicates a booster shot given at least six months after the second vaccine dose produces antibody levels five to 10 times higher than after two doses.

Write to Jonathan D. Rockoff at Jonathan.Rockoff@wsj.com and Jared S. Hopkins at jared.hopkins@wsj.com

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