As with all things related to COVID-19, new data is constantly emerging, but the information we have so far consistently suggests that breakthrough infections are relatively rare, says Timothy Brewer, MD, professor of medicine and epidemiology at UCLA. And importantly, the majority of the post-vaccinated infections that do occur tend to be mild to moderate rather than severe or life-threatening.
Dr. Brewer points to data recently published in the New England Journal of Medicine that evaluated breakthrough infections among healthcare workers in Israel. "There, it was occurring on the order of about half a percent," he says. "There were 39 breakthrough infections in a population of 1,497, and all of the infections were mild to moderate." He also says that there were no noted cases of secondary transmission from those who had breakthrough infections in this group, which means the data didn't indicate that they passed it along to others. These findings are similar to those from studies out of other countries, says Dr. Brewer. He points to a pre-print study that followed 24,000 healthcare workers in the Netherlands and found breakthrough infections occurring at under one percent. Another pre-print study on 3,720 healthcare workers in Italy found breakthrough infections occurring at a rate of under one percent as well.
Dr. Brewer explains that there are pros and cons to research that focuses specifically on healthcare workers. The upside is that they were vaccinated ahead of the general population, so if immunity from the vaccines wanes over time, their's would be the first to do so. This population is also easy to monitor because they get tested often, Dr. Brewer says. Still, extrapolating data from healthcare workers and applying it to the general population can be imprecise because healthcare workers tend to be relatively young (e.g., in their thirties and forties) and healthier than the general population, says Dr. Brewer. For these reasons, they may have better outcomes than would more diverse population samples.
However, a new study published in The Lancet included a more general population sample in the UK and offers similarly encouraging results. Out of 1.2 million people who received two doses of a vaccine, the study found that only 0.5 percent reported a breakthrough infection two weeks or more after their second shot. This data was collected between December of 2020 and July of 2021, so it's unclear whether or not infections became less rare the further out individuals became from their second dose.
Additionally, the Centers for Disease Control and Prevention (CDC) switched from reporting all breakthrough infections to monitoring cases that led to hospitalization and death. However, the Kaiser Family Foundation analyzed data from 24 states that surveil COVID-19 breakthrough infections and found these cases occurred in less than one percent of vaccinated individuals.
So if the percentage of breakthrough infections remains relatively low, why does it feel like everyone knows someone who got COVID-19 after vaccination? Dr. Brewer calls this "selection bias" and says that knowing someone who has had a breakthrough infection doesn't tell you anything about the rate of breakthrough infections—it just tells you that they happen, and epidemiologists and public health officials always knew they would occur (because none of the vaccines are 100 percent effective at preventing infection).
"Nobody walks around saying, 'Everybody in my neighborhood has had a breakthrough infection,'" he says. Instead, he explains, they might say they know one vaccinated person in their neighborhood who had a breakthrough infection, but what they're not saying is that they know 100 total vaccinated people in their neighborhood with COVID-19. This would mean that, as far as they know, 99 of the people they live alongside did not have a breakthrough infection. In other words, anecdotes are not evidence of a widespread issue. It may feel like "everyone" is getting infected, but that's not empirically true. Breakthrough infections are occurring, but they remain statistically rare, and again, these infections tend to be mild to moderate.
This isn't to say that breakthrough infections aren't without risk, however. Dr. Brewer says we have somewhat limited data on the risk of long-haul symptoms resulting from breakthrough infections, but data suggest that long-haul COVID risk is halved by full vaccination. "So presumably, the risk of long haul is still lower in vaccinated individuals than in unvaccinated individuals," says Dr. Brewer.
As is the case with COVID-19 infection in unvaccinated populations, your chance of contracting more severe illness if infected post-vaccination increases based on your risk factors or vulnerabilities. Older individuals, those with underlying health conditions, and those who are immunocompromised are at a greater risk of having complications from a breakthrough infection, even if they're fully vaccinated. Boosters are expected to help better protect these populations and reduce breakthrough cases overall.
But Dr. Brewer notes that more of the world's population must be immunized, or we will just be playing "whack-a-mole" with new variants that challenge vaccines and lead to additional breakthrough infections. Getting vaccinations approved for children under the age of 12 will be hugely helpful, too, and Dr. Brewer says he's hopeful that trial data on the safety and efficacy of vaccines for young children will be available this fall so that inoculations can be made available soon thereafter. This will help stop kids from spreading the virus and enabling breakthrough infections in their vaccinated family members, teachers, and others with whom they come into contact.
In the meantime, your risk of getting infected despite vaccination is higher if you live in an area with high virus circulation and low vaccination rates. It also depends on the risks you take, and Dr. Brewer advises wearing a mask indoors, avoiding large crowds, and practicing other preventative measures such as regular hand washing. "But just to reiterate—breakthrough infections are still uncommon, and they tend to be mild or moderate when they do occur," he says. "Getting vaccinated still remains the best way to protect yourself and those around you."
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