What is the chance of a “breakthrough” infection after vaccination?

Infection and Spread Vaccines

A: Quite low, but not zero.

The U.S. Centers for Disease Control and Prevention (CDC) recently released data on reported breakthrough cases—meaning a positive SARS-CoV-2 test more than 2 weeks after FULL vaccination.


➡️ 5,814 cases were reported out of over 75 MILLION people who were fully vaccinated by this time.
➡️ 396 people were hospitalized, but 133 of these were hospitalized for reasons other than COVID-19 (they happened to test positive in hospital).
➡️ 74 people died, of which 9 died due to a cause unrelated to COVID-19.

❓What do these numbers mean?

5814/75,000,000 = 0.0077% … a very small number. This is very encouraging.

Unfortunately, we can’t directly compare these numbers to the efficacy numbers calculated in clinical trials, because we don’t have a control group to see the difference in infections between vaccinated and unvaccinated people.

❓Do these breakthrough cases mean the vaccines don’t work? No–we already knew from the trials that breakthrough infections would occur.

Remember the amazing 95% efficacy of the Pfizer trial? Out of 18,198 fully vaccinated people in the trial, there were 8 positive symptomatic SARS-CoV-2 cases (compared to 162 in the similarly sized placebo group). This was 95% fewer cases in the vaccine vs. placebo group, but it still means we can expect infections to occur, especially when millions of people are vaccinated.

The CDC will be following these data on breakthrough infections for any patterns that might point to people at increased risk of infection after vaccination, but so far, no major patterns were identified. 45% of the reported infections were among people ≥60 years of age, but older people were more likely to be fully vaccinated by this time, so it doesn’t necessarily mean they are more likely to get infected after vaccination.

Even though these numbers are extremely low, they are likely underestimates. First, some of the 75 million in the denominator were only recently fully vaccinated so didn’t have time to become infected. Second, many asymptomatic cases are unlikely to be tested. See our recent post for studies that have followed vaccinated people over time AND tested for asymptomatic infection (the news is also GOOD!).

Hospitalizations and deaths were even rarer than infections after vaccination, but they did occur. The trials were much smaller (~40,000), and almost no cases of hospitalization and death were seen in the vaccine groups. When you vaccinate millions of people, rare events such as severe breakthrough infections and deaths will happen. This is sad, as we all want to believe that we and our loved ones are 100% bulletproof after vaccination. While not 100%, our chance of severe COVID-19 or death is *dramatically* lower than it would be unvaccinated.

☔If a mask is like an umbrella that can keep you dry in light rain but not a driving storm, the vaccine is like a giant outdoor tent. You’ll almost always be dry except for extreme cases- like a hurricane that blows the tent away 🌀.

❓How can we reduce the number of breakthrough infections? It’s in our hands!

The number of breakthrough infections depends heavily on the level of transmission (or rain) around you—if there is very little virus circulating there is hardly any chance for you to have a breakthrough infection (not much rain coming down). But if transmission is super high, the howling winds and rain will be pulling up the stakes of that tent…and you may get wet.

So use your #StaySMART prevention tips a bit longer, avoiding crowded indoor spaces and using a mask when you can’t avoid close contact in public. As vaccines go ⬆️ & cases go ⬇️ the better protected *everyone* will be.

Stay Safe and Stay Sane!
Those Nerdy Girls

Further reading:

Summary of CDC data and systems for monitoring breakthrough infections

“You’re Vaccinated. Cool! Now About Those ‘Breakthrough’ Infections…”

Link to original FB post