COVID-19: Vaccine Immunity, Boosters, and Breakthrough Cases Explained

Published on: Apr 29, 2021fb-roundtwitter-roundemail-round
A woman not wearing a face mask speaks on the phone in Washington, DC.Getty Images

As of April, more than 20% of the US population has been fully vaccinated against COVID-19. And some people have been vaccinated for months –  receiving their first shot as far back as December, when COVID-19 vaccines first debuted in the US. Now that the country has been administering doses for several months, some are wondering about how long its benefits will last.

Here’s what we know about COVID-19 vaccine immunity so far… 

While we don’t know exactly how long immunity from the shots will last, many experts say they probably won’t give people lifelong protection (like a measles vaccine does). Dr. Anthony Fauci said that it’s “highly likely” that a vaccine will be effective for longer than six months. But that people may need a booster shot in as little as a year. Here’s what we’ve heard from Pfizer, Moderna, and Johnson & Johnson – the companies whose vaccines have been approved for emergency use in the US – so far...

  • Pfizer’s about 95% effective at preventing COVID-19. And the company said it was still 91% effective after six months. But in April, Pfizer’s CEO announced that people will “likely” need a third dose within 12 months of being fully vaccinated – and after that people will probably need an annual dose (like the flu shot). 

  • Moderna’s about 94% effective at preventing COVID-19. And studies show after six months, the vaccine is still more than 90% effective. Moderna’s CEO says that the company hopes to have a booster ready by the fall to protect people through winter. He also said he believes that annual COVID-19 boosters will become the norm.

  • Johnson & Johnson’s about 72% effective in the US (though studies show it seemed to be less effective globally). The company has reportedly said that people will probably need to get its single-dose vaccine once a year.

Here’s why else you might need a booster...

Variants. As the virus continues to spread, it makes copies of itself – but sometimes, not all of the same instructions make it into the new copy. So it becomes a different version, aka a new strain or variant. Right now, there are a few spreading across the US. While data suggests that the current vaccines offer protection against most variants, we still don’t know how they work on every variant that exists today – or ones that could develop in the future.

Now, variants are contributing to the spread of COVID-19 in the US, where there are more than 50,000 new cases reported a day. That number is high despite millions of Americans getting vaccinated a day. The CDC says variants will cause some people to get the coronavirus – even if they’re fully vaccinated.

Here’s what you need to know about breakthrough cases… 

Breakthrough cases are rare and occur when a fully vaccinated person contracts COVID-19. CDC data shows that as of mid-April, there were about 5,800 of these cases reported out of 77 million people who were fully vaccinated by that point. Twenty-nine percent of these breakthrough cases showed no symptoms, while 7% were hospitalized and about 1% died.

Breakthrough cases have been linked to nursing homes in Chicago and Kentucky. But they can occur in people of all ages. Reminder: vaccines help prevent the spread of COVID-19 and prevent severe symptoms – but they are not a cure for the disease.

While all three authorized COVID-19 vaccines are highly effective, none prevent illness 100% of the time. And other factors outside of variants can cause breakthrough cases, including timing. It takes a couple of weeks for your body to build up immunity, so you could contract the virus right before or right after you get your shot.


The COVID-19 vaccines are essential in protecting people against the virus, and they could help us get back to life pre-pandemic. And as the data continues to roll in and more people get their shots, it’s still important to practice the basic safety precautions, even if you’re fully vaccinated.

Skimm'd by Maria Martinolich and Kamini Ramdeen

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