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

Published on: Aug 18, 2021fb-roundtwitter-roundemail-round
A nurse administers a dose of the Pfizer Covid-19 vaccine to a college student at the California State University Long Beach (CSULB) campus on August 11, 2021.Getty Images

As of mid-August, about 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 the country has been administering doses for more than half a year. And some are wondering how long its benefits will last.

Here’s What To Expect When It Comes To Booster Shots… 

In August, the CDC recommended that those who are moderately or severely immunocompromised get an additional vaccine dose, including people who have…

  • Been getting active cancer treatment for tumors or cancers of the blood

  • Gotten an organ transplant and are taking meds to suppress their immune system

  • Gotten a stem cell transplant within the last two years or are taking meds to suppress their immune system

  • Moderate or severe primary immunodeficiency

  • Advanced or untreated HIV infection

  • Active treatment of drugs that could suppress your immune system 

The Biden admin has also announced that those who got a Pfizer or Moderna vaccine (aka a two-dose regimen) can get a booster shot starting the week of Sept 20. Shots are expected to go to more at-risk populations first (think: people 65 and older, health care workers). And people should wait about eight months after their second dose to get it. But the FDA still needs to sign off on the third dose before this plan can go into action. Officials say that those who got a Johnson & Johnson shot will likely need a booster as well, though officials are waiting on more data to make a call.

A number of countries, including France, Germany, and Israel have offered or plan to offer booster shots to certain people. But in early August, the World Health Organization asked countries to ‘hold the boosters.’ That’s because many living in poorer countries haven’t had a chance to get even one dose yet. Our World in Data shows that only 1.3% of people in low-income countries have gotten at least one shot.

The Delta Variant’s Role

The delta variant accounts for about 98.8% of new coronavirus infections in the US. The CDC says that it’s more contagious than other strains – and that fully vaccinated people who experience breakthrough cases (more on that below) can spread the virus. But, the agency also noted that the vaccines authorized in the US are highly effective, including against this variant.

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. About 125,000 fully vaccinated Americans have had a breakthrough case, according to data from NBC News in late July. There were 8,054 breakthrough cases in fully vaccinated people that resulted in hospitalization or death, according to the CDC as of Aug. 9. As of that date, more than 166 million Americans were fully vaxxed.

Reminder: Vaccines help prevent the spread of COVID-19 and prevent severe symptoms – but they are not a cure for the disease and do not 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. If you are exposed to or test positive for COVID-19 after getting jabbed, here’s what you need to know to keep yourself and others safe.


The COVID-19 vaccines are essential in protecting people against the virus, and have helped us get back to pre-pandemic life. But coronavirus variants are throwing a wrench in those plans, causing new spikes and breakthrough cases around the world. Officials hope that booster shots can give people an added layer of protection. But 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 August 18 – Updated to include details on the US offering booster shots and the latest numbers on breakthrough cases.

Updated on August 4 – Updated to include details on countries administering booster shots and the latest numbers on breakthrough cases.

Skimm'd by Maria Martinolich and Kamini Ramdeen

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