Got questions about COVID vaccine booster? Here’s some answers.

U.S. health officials on Wednesday recommended that all U.S. vaccinated residents should get a COVID-19 booster shot — even as 40 percent of the population has yet to receive their first dose.

Americans who received either the Pfizer or Moderna vaccines should get a booster dose eight months after they received their second shot of the two-dose mRNA vaccines, according to the U.S. Department of Health and Human Services. About 154 million U.S. residents are eligible for the booster, federal data shows.

That recommendation comes as daily COVID-19 cases have surged from about 10,000 a day in early July to more than 150,000 infections a day this month, severely taxing hospital systems in the hardest-hit states, including Florida.

More than 90 percent of those cases are believed to be from the highly infectious delta variant of the virus. The vaccinated can catch and transmit the variant in some cases, but vaccines help shield them from the worst symptoms. The unvaccinated have no such protections, however, and account for most infections and deaths in the U.S.

Concern about the delta variant prompted the Centers for Disease Control and Prevention on Tuesday to recommend a third dose for around 9 million Americans whose immune systems are moderately to severely compromised and may not be fully-protected by a two-dose vaccine series. That population accounts for about 40 percent of all breakthrough cases, according to the federal agency.

Booster shots for the rest of the population could be rolled out as soon as Sept. 20. That schedule is subject to the Food and Drug Administration evaluating the safety and effectiveness of a third dose. The CDC’s Advisory Committee on Immunization Practices must also review the data and issue its own booster recommendations.

The booster roll out is expected to follow the same order as the first vaccination shots, with frontline health care workers, nursing home residents and older Americans getting priority.

Here’s what you need to know about the booster:

Who should get a booster shot and why?

The booster vaccine dose is recommended for people who received one of the the mRNA vaccines manufactured by Moderna and Pfizer. It should be given about eight months after the second dose.

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The booster is needed, officials said, because studies in the U.S. and abroad have shown that the level of COVID antibodies — the cells that attack the virus — decrease over time. That’s normal for any vaccine. For example, children routinely receive a Hepatitis booster shot six months after the initial vaccination. Tetanus boosters are recommended every 10 years.

Experts said that the decision does not mean the vaccines are ineffective. The vast majority of people admitted to hospitals during the recent surge in COVID cases have been unvaccinated.

However, there has been evidence that over time the vaccine is providing “reduced protection against mild and moderate disease” from the highly infectious delta variant, the U.S. Department of Health and Human Services said.

Is the booster shot different from the first two doses?

No, the booster is the same formulation as the first two vaccine doses already administered to millions of Americans and is intended to rev up the immune system against COVID. For people with weakened immune systems, the extra dose is classified as a third shot because the first two doses may not be enough to fully protect them. They’re also free, just like the other vaccine shots.

What about people who received the Johnson & Johnson vaccine?

U.S. health officials say the roughly 14 million recipients of the one-dose Johnson & Johnson vaccine will likely need a booster too. However, that vaccine was not authorized for use until March, so scientists are still studying the data and have not yet issued a recommendation.

If I received the Moderna vaccine, should I get a Moderna booster?

The CDC says the third shot should be the same kind of vaccine as the first two doses. If that is not feasible, then an additional dose using the other mRNA vaccine is permitted.

Does the booster protect against the delta variant?

Scientists believe the vaccines are highly effective at protecting people from developing severe COVID symptoms, including from the delta strain. But the vaccines were developed before scientists were aware of the delta variant and data suggests that, over time, there is a decline in how well they protect against less serious symptoms.

A recent study of a delta outbreak in small Massachusetts town with a high vaccination rate showed that vaccinated individuals carried as much virus in their noses as unvaccinated ones, suggesting they could still spread the virus even if they had no symptoms. That study was a key reason why the CDC updated its mask recommendation and asked vaccinated people to again wear masks indoors.

A higher antibody response created by the booster will improve the body’s ability to quickly fight the virus and should lower the chance of similar transmission, said Dr. Jason Wilson, associate medical director of the adult emergency department at Tampa General Hospital.

“That was a big red flag,” he said of the Massachusetts study. “We need to up the actual war here against this viral strain that is more transmissible.”

I got vaccinated, then caught COVID. Do I still need a booster?

Yes. A CDC study in May found that unvaccinated people who caught COVID were twice as likely to be re-infected compared to recovering COVID victims who later got vaccinated. University of Florida epidemiologist Thomas Hladish points to reports of people infected multiple times as evidence that the immunity provided by vaccines lasts longer and is more effective than the natural immunity gained from contracting COVID.

What about the unvaccinated and achieving “herd immunity”?

Experts agree that the best way to control the pandemic is to convince unvaccinated people to get immunized. Some worry that the booster recommendation will make that tougher, because vaccine skeptics will see it as another reason not to get the shot.

University of South Florida immunologist Michael Teng said he still receives emails from people who are vaccine-hesitant and need reassurance that they are safe and effective. The booster recommendation may deter them, he said, which is why he prefers the country focus on getting as many people vaccinated as possible.

“It brings into question how good these vaccines are,” he said. “That might knock some of the vaccine hesitant people off the fence the wrong way.”

The emergence of the contagious delta variant, which can be spread by vaccinated people, has likely moved the goalposts for reaching herd immunity anyway, Hladish said.

But there is still merit in the booster shot, he said. It will increase protection against severe COVID symptoms and could make it less likely that fully vaccinated people spread the virus. That could help slow the spread of COVID even if the rate of people receiving their first dose continues to rise slowly.

“Epidemics are like fires,” he said. “If we can do something about the virus’ ability to infect vaccinated people, that is still taking away the fuel for the pathogen.”

Are COVID vaccines safe?

Wilson of Tampa General Hospital said vaccines have a proven history of safely eradicating infectious diseases. Serious side effects like Guillain-Barré Syndrome were linked to just 100 cases out of more than 13 million Johnson & Johnson doses. More than 357 million doses have been given out across the U.S., according to the CDC.

“We have millions of people who have been vaccinated and very good we have very good surveillance data,” he said.

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