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

Published on: Jul 12, 2021fb-roundtwitter-roundemail-round
A woman not wearing a face mask speaks on the phone in Washington, DC.Getty Images

As of mid-July, nearly half of the US population has been fully vaccinated against COVID-19. And some 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 more than half a year, some are wondering how long its benefits will last.

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

Experts are still looking into how long immunity from the shots will last. But for now, it seems like a booster may not be needed. In July, Dr. Anthony Fauci said those who are fully vaccinated do not need a booster shot “right now.” But there’s research being done into whether they’ll need one eventually.

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). For now, experts are torn on whether people will need a booster – and if a booster could even be a dose of Pfizer or Moderna. The J&J shot has come under renewed scrutiny: in July, the FDA said it’s linked to a rare neurological disorder called Guillain-Barré syndrome. For context: the CDC has confirmed about 100 cases of this out of the 12.8 million J&J doses that have been administered.

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 and the world, including the highly contagious delta variant.

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. And even though vaccinations in the US have helped curb the virus, the delta variant has caused an uptick in a number of states.

Let’s look at Israel – one of the fastest countries to inoculate its citizens. Recently, the country has seen a spike in cases caused by the delta variant. One study from Israel found that the Pfizer shot is 64% effective against the variant – down from 94%. (Worth noting that the study found the vaccine still prevents severe illness, and the US has not confirmed that data). And in July, the country started administering a third Pfizer COVID-19 vaccine to immunocompromised adults in an effort to better protect them from the virus. Pfizer said it plans to seek authorization for a booster in the US and EU – and reps are meeting with US officials in July to discuss this.

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

Breakthrough cases are rare and occur when a fully vaccinated person contracts COVID-19. And they can happen to people of all ages. CDC data shows that as of mid-July, there were 5,186 breakthrough cases that resulted in hospitalization or death. That’s out of more than 157 million people who were fully vaccinated by that point.

Reminder that vaccines help prevent the spread of COVID-19 and prevent severe symptoms – but they are not a cure for the disease. While all three COVID-19 vaccines authorized in the US 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 have helped us get back to life pre-pandemic. But coronavirus variants are throwing a wrench in those plans, causing spikes in cases around the world. 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.

Updated on July 12  – Updated to include the latest information on booster shots and breakthrough cases.

Skimm'd by Maria Martinolich and Kamini Ramdeen

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