What You Need to Know About Catching COVID-19 After Vaccination
If you’ve been fully vaccinated against COVID-19, maybe you figured you no longer need to worry about contracting the coronavirus. But along with the rising number of new COVID-19 cases globally and growing concern about highly transmissible strains like the delta variant come reports of fully vaccinated people testing positive for COVID-19.
Members of the New York Yankees, U.S. Olympic gymnast Kara Eaker and U.K. health secretary Sajid Javid are some of those diagnosed with what is called a “breakthrough infection.”
As scary as the term may sound, the bottom line is that the existing COVID-19 vaccines are still very good at preventing symptomatic infections, and breakthrough infections happen very rarely. But just how common and how dangerous are they? Here’s a guide to what you need to know.
What is ‘breakthrough infection?’
No vaccine is 100% effective. Dr. Jonas Salk’s polio vaccine was 80%-90% effective in preventing paralytic disease. Even for the gold standard measles vaccine, the efficacy was 94% among a highly vaccinated population during large outbreaks.
Comparably, clinical trials found the mRNA vaccines from Pfizer and Moderna were 94%–95% effective at preventing symptomatic COVID-19 – much more protective than initially hoped.
A quick reminder: A vaccine efficacy of 95% does not mean that the shot protects 95% of people while the other 5% will contract the virus. Vaccine efficacy is a measure of relative risk – you need to compare a group of vaccinated people to a group of unvaccinated people under the same exposure conditions. So consider a three-month study period during which 100 out of 10,000 unvaccinated people got COVID-19. You’d expect five vaccinated people to get sick during that same time. That’s 5% of the 100 unvaccinated people who fell ill, not 5% of the whole group of 10,000.
When people get infected after vaccination, scientists call these cases “breakthrough” infections because the virus broke through the protective barrier the vaccine provides.
How common is COVID-19 infection in the fully vaccinated?
Breakthrough infections are a little more frequent than previously expected and are probably increasing because of growing dominance of the delta variant. But infections in vaccinated people are still very rare and usually cause mild or no symptoms.
For instance, 46 U.S. states and territories voluntarily reported 10,262 breakthrough infections to the U.S. Centers for Disease Control and Prevention between Jan. 1 and April 30, 2021. By comparison, there were 11.8 million COVID-19 diagnoses in total during the same period.
Beginning May 1, 2021, the CDC stopped monitoring vaccine breakthrough cases unless they resulted in hospitalization or death. Through July 19, 2021, there were 5,914 patients with COVID-19 vaccine breakthrough infections who were hospitalized or died in the U.S., out of more than 159 million people fully vaccinated nationwide.
One study between Dec. 15, 2020, and March 31, 2021, that included 258,716 veterans who received two doses of the Pfizer or Moderna vaccine, counted 410 who got breakthrough infections – that’s 0.16% of the total. Similarly, a study in New York noted 86 cases of COVID-19 breakthrough infections between Feb. 1 and April 30, 2021, among 126,367 people who were fully vaccinated, mostly with mRNA vaccines. This accounts for 1.2% of total COVID-19 cases and 0.07% of the fully vaccinated population.
COVID Testing Nasal Swab
Even if you’re fully vaccinated, you should get tested if you have symptoms.
How serious is a COVID-19 breakthrough infection?
The CDC defines a vaccine breakthrough infection as one in which a nasal swab can detect the SARS-CoV-2 RNA or protein more than 14 days after a person has completed the full recommended doses of an FDA-authorized COVID-19 vaccine.
Note that a breakthrough infection doesn’t necessarily mean the person feels sick – and in fact, 27% of breakthrough cases reported to the CDC were asymptomatic. Only 10% of the breakthrough-infected people were known to be hospitalized (some for reasons other than COVID-19), and 2% died. For comparison, during the spring of 2020 when vaccines were not yet available, over 6% of confirmed infections were fatal.
In a study at U.S. military treatment facilities, none of the breakthrough infections led to hospitalization. In another study, after just one dose of Pfizer vaccine the vaccinated people who tested positive for COVID-19 had a quarter less virus in their bodies than those who were unvaccinated and tested positive.