New research about breakthrough coronavirus infections finds they may be less likely to spread to others because vaccinated people don’t shed the virus for as long as the unvaccinated.
The study, which involved NBA players, their families and staff, found that people with breakthrough COVID-19 cases stopped producing the virus two days sooner than the unvaccinated. The vaccinated people who got COVID-19 cleared the virus from their systems on average in 5½ days. The unvaccinated took 7½ days.
The researchers took 19,941 viral samples from 173 National Basketball Association players, staff and their families between November 2020 and August. They compared the viral dynamics of those infected with various coronavirus variants and the course of illness in players who were vaccinated and unvaccinated.
“Most of them were getting tested every day or every other day,” said Stephen Kissler, an infectious disease fellow at the Harvard T.H. Chan School of Public Health and co-author on the paper. “This allowed us to uncover what infections look like in the very early stages. Usually, we only see the virus once a person has symptoms, when they’re at peak.”
The study, published Wednesday in the New England Journal of Medicine, also found that different coronavirus strains produce about the same amount of the virus, which hadn’t been clear before.
There had been some early research suggesting the delta coronavirus variant produced as much as 1,000 times as much virus as other variants.
“It seems to be not so much that people with delta are producing huge amounts of virus but that the virus is better able to infect our cells,” he said.
Kissler cautioned that even if vaccinated and unvaccinated people produce the same amount of viral material, that doesn’t necessarily represent how capable they are of causing infection.
“In a person who’s been vaccinated, in the viral genetic material they produce, the virus is chopped up and weakened, because their immune system is fighting the virus, so it’s less likely to be infectious,” he said.
Genomic analysis of breakthrough infections
More information about how breakthrough cases work is becoming available as the pandemic progresses and researchers investigate the differences between vaccinated and unvaccinated COVID-19 patients.
The biggest contrast, of course, is that people who are unvaccinated are 5.8 times more likely to get COVID-19 than vaccinated people. The unvaccinated are 14 times more likely to die from COVID-19 than people who are vaccinated.
Being vaccinated is also more protective than having recovered from an earlier case of COVID-19 and having what is known as natural immunity.
A large study by the Centers for Disease Control and Prevention published in October found that unvaccinated people who had had a recent infection were five times more likely to get COVID-19 than those who were fully vaccinated and had not had a prior infection.
There are also some indications the virus must work harder to infect vaccinated people. A study at the University of California, San Francisco found breakthrough infections among vaccinated people were more commonly associated with slightly more antibody-resistant strains of the coronavirus than those seen in unvaccinated people.
“Over time there’s some selection going on,” said Dr. Charles Chiu, an infectious disease specialist at UCSF and lead author on the paper, whose lab on Wednesday identified the first case of the omicron variant in the U.S.
Researchers performed whole-genome sequencing of virus samples from 1,373 infected people between February and June. Among the vaccinated, 78% of infections showed at least one mutation associated with a decrease in COVID-19-neutralizing antibodies. Among the unvaccinated, 48% showed such mutations.
Breakthrough infections appear to be happening in part because the virus is mutating to be better at evading immunity. The mutations are minor, but they are happening.
Breakthrough infections were also more likely to be found in people with low or undetectable neutralizing antibody levels, either because they were immunocompromised or because they were infected with an antibody-resistant linage of the virus, Chiu said.
The researchers found that while the viral load was effectively the same between vaccinated and unvaccinated people, there was a difference between symptomatic and asymptomatic breakthrough infections.
Vaccinated people who had symptomatic breakthrough case had viral loads similar to those of unvaccinated people. But vaccinated people who had coronavirus infections but no symptoms of COVID-19 had significantly lower viral loads.
Some research has indicated higher viral loads are more likely to be associated with infectiousness, so vaccinated people with asymptomatic COVID-19 may be less likely to infect others. And researchers in the United Kingdom found that people with breakthrough infections are two times more likely to be asymptomatic than cases in the unvaccinated.
Overall, the term “breakthrough” is unfortunate because it doesn’t make clear that most infections among the vaccinated result in mild disease, said John Grabenstein, a former senior scientist and director for the U.S. Department of Defense military immunization program.
“Severity is reduced and transmission risk is reduced, even if not to zero,” he said. “And that’s good.”
