Breakthrough infections — fully vaccinated people testing positive for COVID-19 — seem to be cropping up everywhere, including the Tokyo Olympic Games, the White House and Congress.
Public health officials say such infections may be contributing to the spread of the coronavirus, although unvaccinated people account for most of the surge of COVID-19 cases. They point to the virulence of the coronavirus’s delta variant, which first emerged in India.
“What we have learned with the delta variant, which is so highly contagious with an awful lot of spread among the unvaccinated, there’s been spillover into people who are vaccinated, and we’re seeing more so-called breakthrough infections,” said Dr. William Schaffner, an infectious diseases professor at Vanderbilt University.
“One of the things we’ve learned is that, although people shed less virus when they are vaccinated, some of those vaccinated people can be transmitters. And that has become more apparent with the delta strain,” he said.
Dr. Schaffner said people infected with the delta variant have more viral load and exhale much more of the virus, although estimates of the strain’s transmissibility vary. At worst, the delta variant could produce 1,000 times more viral load in the throat, he said, citing a Chinese study.
In a COVID-19 update Wednesday, Maryland Gov. Larry Hogan cited data showing that the delta variant is 225% more transmissible than the original coronavirus strain, putting the unvaccinated in particular at a higher risk.
Although the majority of transmission is coming from the unvaccinated, vaccinated people infected with the delta variant might be spreading the virus to others “on rare occasions,” Dr. Rochelle Walensky, director of the Centers for Disease Control and Prevention, said this week.
The CDC on Tuesday revised its mask guidance. It now recommends that vaccinated Americans wear face coverings in schools and public indoor spaces in areas with high transmission of the coronavirus, partially because of data showing that fully inoculated people may be able to spread the virus.
Six Democrats in the Texas Legislature who fled to Washington to prevent lawmakers from voting on an election bill have tested positive for coronavirus infection, although they were vaccinated.
Breakthrough infections also have been reported in a White House official and an aide to House Speaker Nancy Pelosi, along with several congressional staff members.
Before the Olympic Games began, U.S. gymnast Kara Eaker tested positive for COVID-19 at a training camp outside Tokyo even though she said she was vaccinated, The Associated Press reported. WNBA player Katie Lou Samuelson also tested positive despite being inoculated. She withdrew from the Olympics’ 3-on-3 basketball competition.
As of July 19, the CDC received reports of 5,914 patients with breakthrough infections who were hospitalized or died. More than 161 million people in the U.S. had been fully vaccinated.
Of those breakthrough infections, 74%, or 4,392 patients, were 65 or older. Twenty percent, or 1,164, experienced asymptomatic infections. The CDC reported 1,141 deaths among those with severe vaccine breakthrough cases, but 26% of these cases were reported as asymptomatic or not related to COVID-19.
The number of COVID-19 vaccine breakthrough cases is probably higher than what has been reported to the CDC, the agency says, but the infections affect only a small percentage of inoculated people. Although coronavirus vaccines are effective, they do not prevent illness in vaccinated people at a rate of 100%, the CDC notes.
Mr. Hogan said only 0.07% of Maryland’s entire vaccinated population has had COVID-19 breakthrough infections, demonstrating a 99.93% vaccine effectiveness.
“Breakthrough infections occur because the vaccine is not a big zapper or force field. Tiny bits of viral genetic material being found in the nose is usually not clinically significant, rare, and not evidence of the vaccine failing,” said Dr. Amesh Adalja, senior scholar for Johns Hopkins Center for Health Security.
“On the contrary, it is evidence of the vaccine working as it is severely blunting the impact of infection. Breakthrough disease is exceedingly rare and hospitalization even rarer,” Dr. Adalja said. “The primary aim of the vaccines is to prevent serious disease, hospitalization and death. They do that tremendously.”
Compared with people who haven’t been vaccinated, those with breakthrough infections could experience mild or no symptoms, experts say.
Dr. Schaffner said these infections often sound like a “bad cold” for people who are vaccinated. He said some research and anecdotal evidence suggest reduced symptoms such as cough and loss of taste and smell among vaccinated people who become infected.
He said COVID-19 vaccines prevent severe disease and that most breakthrough infections are mild and don’t require hospitalization.
Clinical evidence does not appear to point to a greater frequency of breakthrough infections with any of the vaccines available in the U.S., Dr. Adalja said, but infections might be more common with Chinese vaccines, as reported in the Seychelles.
Dr. Adalja suspects breakthrough infections might become more common with the length of time since vaccination. He added that it is not surprising and “not a major concern” as long as the vaccination protects people from severe disease.
Breakthrough infections are more likely from the delta variant, the dominant strain now circulating in the U.S. The amount of viral exposure could be higher because of the strain’s “increased fitness,” he said. The beta variant has also reportedly caused breakthrough infections.
Dr. Adalja recommends that those who are immunocompromised and in high transmission areas wear masks even if they are fully vaccinated.
“For others, I think it’s about risk tolerance because breakthrough infections are not clinically significant and are not driving infections in this country,” he said. “The goal of the vaccine program was not to eliminate COVID-19, which is impossible, but to reduce its ability to cause serious disease, hospitalizations and death; to deny the ability to threaten hospital capacity; to make it more like other respiratory viruses as we deal with year in and year out.”
If most breakthrough infections are mild, then vaccine booster shots likely are unnecessary, Dr. Schaffner said, but if an increasing number of vaccinated people require hospitalization for severe cases, then booster shots might be a good idea.
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