Endless news cycles and viral social media warn of “breakthrough infections” in people already vaccinated for COVID-19. These reports leave the mistaken impression that protections afforded by the vaccines are not working—and they can fuel reticence among the millions of people in the U.S. who have yet to get a shot. But such infections are not only known to occur after COVID vaccination. They frequently happen following inoculation against influenza, measles and many other diseases.

SARS-CoV-2, the virus that causes COVID, is special in one way, though: more than any other pathogen, it has provided the public at large with lessons in immunology—and terms such as “breakthrough infections” and “herd immunity” have gained a broad familiarity. “It almost feels not just like a microscope but an electron microscope on every single thing that happens with the COVID vaccines,” says Kawsar Talaat, an associate professor in the department of international health at the Johns Hopkins Bloomberg School of Public Health. No vaccine is 100 percent effective, she notes, and “although some are better than others, most of them have some breakthrough infections.”

A “breakthrough” simply means that a vaccinated person has tested positive for the disease-causing agent, not that they will become ill or transmit the infection to someone else. Most vaccinated people who are infected do not have symptoms, and those that do tend to have mild illness. Even with the Delta variant of SARS-CoV-2, the vaccines show good protection against symptomatic disease and death.

Nationally, as of August 2, the U.S. Centers for Disease Control and Prevention reported that more than 164 million people have been fully vaccinated, just under half of the total population. Yet 97 percent of those who are being hospitalized for COVID-19 are unvaccinated.

The numbers underscore how reality sometimes becomes distorted in the public consciousness. “Anecdotally, from talking to my friends and family and on social media, I think people are more concerned about these breakthrough infections than their prevalence would lead you to be,” says Tara Smith, a professor of epidemiology in the College of Public Health at Kent State University.

Another worry with breakthrough cases is passing the virus to others. But people infected “tend to be less likely to transmit, no matter what we’re looking at,” Smith says. “We see this with viruses and bacteria—even with pertussis, one of the reasons that people try to ‘cocoon’ around infants” who cannot initially be vaccinated against that disease. “Cocooning” refers to vaccinating those who spend time with the infant as a protective barrier because the pertussis vaccine is not administered before the age of two months.

COVID vaccines are expected to reduce transmission among those with an asymptomatic breakthrough infection, says Nick Grassly, a professor in the department of infectious disease epidemiology at Imperial College London. “So you already have the fact that you’re immunized and less likely to become infected, and even if you are infected, your risk of transmitting the virus is reduced,” he adds. One reason is that the amount of the coronavirus, its viral load, is lower in such infections, so there is less of it to transmit. How this pattern looks with the Delta variant is not clear. A Centers for Disease Control and Prevention study published in late July pointed to similar viral counts among vaccinated and unvaccinated people. In that study, however, the researchers did not conduct tests to confirm true viral loads or report data on transmission from vaccinated people, and the "unvaccinated" group included people who were partially vaccinated.

Breakthrough cases do not occur because the vaccines are ineffective. Immunity can wane over time, and a vaccine might be less effective for a given pathogen. The measles-mumps-rubella (MMR) vaccine is one example: its protection against measles is strong, but the immunity to mumps it confers is less so, Talaat says.

Even the mighty measles vaccine has a breakthrough infection history. One measles outbreak in the late 1980s that largely involved vaccinated young people led to a policy change calling for two doses instead of one. The first MMR dose confers about 90 percent protection for a lifetime, Talaat says, but the second dose covers about half of the remaining 10 percent. Given the high contagiousness of measles, getting the highest possible coverage is crucial.

Influenza vaccines are the inoculations that are most associated with breakthrough infections. If such cases of flu were tracked as closely as breakthrough SARS-CoV-2 infections, “there would be so many more” of the former, Smith says, because “we know the flu vaccine is not as effective.” Breakthrough COVID cases, she says, are “just another Wednesday,” compared with those that occur with other vaccines.

The COVID vaccines appear to be faring better than those for influenza usually do. The shots neutralize COVID variants quite effectively so far. Grassly says, that, in fact, COVID does not overcome immunity as much as influenza viruses do. And some types of influenza are just better at dodging what human ingenuity throws at them, making for some flu seasons with very low-efficacy vaccines and plenty of breakthrough cases.

Talaat notes that with the flu, “we’re not calling it ‘breakthrough’ but saying, ‘It’s 47 percent effective this year’ or ‘60 percent....’ We talk about efficacy.” But even though influenza vaccines have relatively poor efficacy, she says, they are “better than nothing,” saving lives and preventing hospitalizations.

Breakthrough rates can increase if the vaccinated population is small and there are high case counts in the community. Conversely, high vaccine uptake means that the vaccinated population makes up a larger proportion of overall cases. If almost everyone is vaccinated, any cases that do occur are likelier to be in someone who’s immunized. That was the situation in an outbreak in Massachusetts in which 74 percent of people testing positive were vaccinated in a region where some 69 percent of eligible residents had received shots.

Other factors contribute to an overrepresentation of vaccinated patients with breakthrough infections, including age and health conditions associated with a weakened immune system. Often, the level of immunity for these patients produces a blunted response to the vaccine, so they may be at a higher risk than younger unaffected people who are not immunized.

Similar to those that are routinely administered for pertussis, booster shots for COVID may be needed for people with a suppressed immune system or for gradually waning immunity. Talaat points to reports of good responses to a third COVID vaccine dose in patients who had an organ transplant. France and Israel have already added a recommended third dose for some immunocompromised recipients, and the U.K. is considering doing so. The CDC’s Advisory Committee on Immunization Practices met on July 22 to review data related to booster shots in people who are immunocompromised and concluded that a third dose might be in order for this patient population.

In an earnings call in late July, Pfizer cited unpublished, preliminary results for 23 clinical trial participants that showed increased protection against the Delta variant after a third dose of its mRNA vaccine. In prepared remarks during the earnings call, the company said that it expected to submit its third dose findings to the FDA in August in a bid to obtain emergency use authorization for a booster.

In the meantime, “we don’t know if boosters will work, but if we vaccinate everybody, then that will protect the 2.7 percent of people in this country who are immunocompromised,” Talaat says. “And they won’t have to worry about how well or how badly their immune system works to protect against the virus.”