Breakthrough infections are exceedingly rare. But by tracking them, researchers can uncover viral variants that manage to dodge the immune response.
Tens of millions of people in the United States have now been fully vaccinated against covid-19. These people are seeing friends, eating out, and—in rare cases—getting infected.
But we shouldn’t panic: these kinds of “breakthrough infections” are entirely expected with any mass vaccine rollout.
According to new figures released by the Centers for Disease Control and Prevention, more than 87 million people had been vaccinated in the US as of April 20. Of those, 7,157—0.008%—went on to become infected with SARS-CoV-2. The true number is likely to be slightly higher because mild or asymptomatic infections might be missed or go unreported. But the data are still encouraging. Only a tiny fraction of vaccinated people became infected and an even tinier fraction developed serious illness: just 331 were hospitalized for covid-19, and 77 died of the disease.
New studies published last week show that even in high-risk settings like nursing homes, these breakthrough infections seem to be rare. And when infections do occur, symptoms tend to be nonexistent or mild. What’s more, vaccinated individuals who become infected have lower viral loads than unvaccinated people—meaning they are less likely to transmit the virus.
Still, it’s important to keep monitoring breakthrough infections. All the existing covid-19 vaccines prime the immune system to recognize the spike proteins on the virus’s surface so that when the system encounters the real pathogen, it can fight back quickly. But if the body doesn’t mount a strong immune response to the shot, it won’t be ready to fight the virus. In other cases, the pathogen may have evolved enough to evade the body’s prepared immune response, and the vaccine won’t work as well. This is called immune escape.
Some researchers studying SARS-CoV-2 worry that with so much virus still circulating globally, the virus will have plenty of chances to hit on a winning combination of mutations that allow it to evade the immune response prompted by the vaccine. Tracking breakthrough infections could help them spot worrisome new variants and identify when vaccines are becoming less effective. This might help determine when booster shots are needed or point to more effective vaccine designs.
People who work and live in nursing homes were among the first to be offered the covid-19 vaccine, and some of the first to benefit. Between late December and late March, case numbers in these facilities dropped by 96%. Nursing homes are ideal places for pathogens to spread and cause harm, and vaccines may not work as well because older people’s bodies typically mount a weaker immune response. Even influenza outbreaks can be deadly in these facilities. But the CDC found few breakthrough covid infections.
In one study, researchers analyzed infections in 78 Chicago nursing homes with nearly 8,000 vaccinated residents and 7,000 vaccinated staff. They found more than 600 coronavirus infections, but only 22 of those occurred in fully vaccinated individuals, 12 in residents and 10 in staff. Fourteen were asymptomatic and five caused only mild symptoms. When the team examined specimens from seven of the people with breakthrough infections, they found low levels of the virus. And none of the initial infections led to additional cases, suggesting that these vaccinated people didn’t spread the virus.
And when outbreaks do occur, the vaccines still provide good protection. A second CDC study examined an outbreak in a Kentucky nursing home where just half the staff were fully vaccinated. The outbreak, which began with an unvaccinated staff member, led to 46 covid-19 infections. Out of 71 vaccinated residents, 18 (25%) became infected, two were hospitalized, and one died. The staff fared better. Of the 56 vaccinated employees, four (7%) became infected. Most of those infections were asymptomatic. Only 6.3% of the residents and staff who had been vaccinated developed symptoms, compared with 32% of unvaccinated individuals.
During a nursing home outbreak, “staff and residents are continuously encountering the SARS-CoV-2 pathogen over and over again,” says Meagan Fitzpatrick, who models infectious diseases at the University of Maryland School of Medicine. So seeing such a small number of infections in this type of setting is encouraging
Image Credit: Envato / Amanda Scott
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