Severe breakthrough infections remain very rare, despite rising concerns

There also isn’t enough data to declare one vaccine better than others against severe illness and death, experts say.

From Washington’s Capitol Hill to Tokyo’s Olympic Village, positive COVID-19 tests among some people who’ve been fully vaccinated are raising concerns about so-called breakthrough infections. That in turn has people questioning whether certain vaccines are more effective against newer variants of the coronavirus, or whether the shots’ efficacy could be wearing off with time.

It’s impossible to say how many breakthrough illnesses are happening in the U.S. because of incomplete data and inconsistent tracking. But according to the Centers for Disease Control and Prevention, the rate of severe breakthroughs is exceedingly low: As of July 19, the CDC had received reports of 5,914 COVID-19-related hospitalizations and deaths out of more than 161 million fully vaccinated people in the United States. While it’s unknown whether everyone who got the vaccine was later exposed to the virus, the reported numbers mean that less than 0.004 percent of people who were fully vaccinated are known to have gotten severely sick or died from COVID-19.

“Breakthrough infections” refers to confirmed cases of COVID-19 (including asymptomatic cases) in people who are fully vaccinated, whereas “breakthrough illnesses” describe infections that make vaccinated people sick. Illnesses can happen if people don’t have a strong immune response to the vaccine, if their immunity has waned with time, or if they are exposed to variants that may have evolved to evade antibodies elicited by vaccination, says Robert Darnell, a physician scientist at the Rockefeller University in New York.

No vaccine is 100-percent effective, and breakthrough infections and illnesses happen with any disease. Because viruses constantly evolve, it’s expected for new variants like Delta—now the dominant variant in the U.S.—to get better at evading vaccines, Darnell adds. Even so, all the vaccines authorized for use in the country drastically reduce the severity and risk of death from COVID-19. More than 97 percent of people currently hospitalized with COVID-19 are unvaccinated, according to CDC Director Rochelle Walensky.

“We're still seeing that infection is absolutely higher among unvaccinated persons and lower among vaccinated persons. It's just not zero,” says Stacey Rose, an infectious disease physician at Baylor College of Medicine in Houston. “What I don't want is for… breakthrough infections to make people think, Oh, well, then forget” getting vaccinated. “Your risk of getting the disease is still lower [if you’re vaccinated], even with the Delta variant.”

Why counting breakthroughs is complicated

Although the CDC originally collected reports of mild and asymptomatic breakthrough infections, the agency shifted in May to focusing instead only on breakthrough hospitalizations and deaths.

Without a centralized tally of breakthrough infections anymore, counting has fallen to states, hospitals, and individual institutions, which track data in different ways. That has made numbers hard to compare among different vaccines, says Francesca Torriani, an infectious diseases physician and hospital epidemiologist at the University of California, San Diego.

Some facilities screen for asymptomatic cases. Others test only when there are symptoms, but criteria differ by facility. Areas with lower rates of vaccination and higher rates of infection have more breakthrough infections. Certain people are more vulnerable because of their behavior or immune systems.

And with a pool of still-unvaccinated people to infect, the virus continues to have opportunities to evolve in ways that might allow it to dodge vaccines.

“There wouldn't be any evolution if everybody got vaccinated—the virus would just go away,” Darnell says. “Vaccination is what basically eradicated measles and came close to eradicating polio. We could do the same thing here with COVID. We could be done with this.”

Researchers reported this month that out of nearly 4,000 healthcare workers, first responders, and frontline workers who got regular nose swabs, there were 204 infections. Only five of those who got sick—or 2.4 percent—had been fully vaccinated.

Is one vaccine more effective against breakthroughs?

Some data—and headlines—have suggested that certain vaccines are less effective, especially against newer variants. But information is changing rapidly, there are many variables, and data are often incomplete, says Darnell.

Many states are counting their breakthrough infections, but even at the local level, data is not being documented by vaccine type, says Teresa McCallion, a spokesperson for the Washington State Department of Health. Statistics may be skewed by variations in dosing schedules, among other factors, including differences in the number of people and the populations who have received each type of vaccine.

“We don’t show the data by vaccine brand because [breakthrough] has been associated with all three current authorized vaccines,” McCallion wrote in an email. “These factors make it difficult to directly compare numbers of breakthrough cases among vaccine brands.”

Given the uneven data on breakthroughs by vaccine type, some researchers are looking for clues about effectiveness in other ways. In one still-unpublished study that has yet to be peer reviewed but garnered recent attention, scientists analyzed blood samples from 10 people who had received the single-dose Johnson & Johnson vaccine and 17 who had received the two Pfizer shots. They found a weaker antibody response among the J&J recipients against newer variants, including Delta.

Despite the headlines that followed that study, it’s too soon to draw conclusions from it, Darnell says, especially with its small sample size. Other research has shown conflicting results, he adds.

In a March 2021 analysis of nearly 15,000 healthcare workers who were regularly screened at UCSD and the University of California, Los Angeles, Torriani and colleagues reported only seven positive tests 15 or more days after vaccination—a positivity rate of 0.05 percent. At UCSD, now that the Delta variant is predominant, breakthrough infections remain far less severe than infections in unvaccinated people, and recovery is quicker, Torriani says. Hospitalizations among vaccinated patients have been very rare, and none have died.

The Delta variant is important to watch, experts say. It appears to be better at evading all vaccines, and cases are on the rise around the U.S. Breakthrough infections also suggest that vaccinated people may still be able to transmit the virus, especially the Delta variant, which is particularly contagious, Darnell says.

Still, while there may be differences among vaccines, more research is needed to accurately compare them, the experts say. “A collaboration between industry companies and academic clinician scientists would be really helpful here,” Darnell says. “We need more science.”

As data accumulates, vaccination remains the best way to protect everyone in the community, including children and people who are immunocompromised, Rose says.

Getting vaccinated matters more than picking a specific vaccine, Torriani adds. “Any vaccine is a good vaccine,” she said. “And there's plenty available.”

Editor's Note: This story has been updated to clarify the interpretation of the CDC's data on severe breakthrough illnesses.

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