One of the most nagging questions about the COVID-19 pandemic for public health authorities, policymakers and the public was whether the vaccines stop transmission.
New Centers for Disease Control and Prevention data show that in breakthrough cases of the highly transmissible delta variant, the answer is no.
The newly released report showing that vaccinated people can still be superspreaders drove the recent decision by the CDC to once again recommend masks for vaccinated people indoors where case counts are high or substantial.
The viral load of vaccinated people with breakthrough cases is the same as in unvaccinated people, the CDC said Friday.
"High viral loads suggest an increased risk of transmission and raised concern that, unlike with other variants, vaccinated people infected with Delta can transmit the virus," CDC Director Rochelle Walensky said in a statement. "This finding is concerning and was a pivotal discovery leading to CDC’s updated mask recommendation. The masking recommendation was updated to ensure the vaccinated public would not unknowingly transmit virus to others, including their unvaccinated or immunocompromised loved ones."
Experts say the report shows the importance of getting a shot to prevent hospitalizations and deaths as the virus continues to circulate.
“The biggest takeaway is that we need more people to be vaccinated. The vaccines are still extremely effective against delta,” said Rachael Piltch-Loeb, a public health emergency preparedness and response fellow at Harvard T.H. Chan School of Public Health. “There is a significant difference between having a fever and dying from COVID-19.”
Another CDC report released Friday shows that the COVID-19 vaccines are typically safe for preteens and teens. Of 8.9 million young people who got a shot, only 863 serious events were reported as possible side effects, a miniscule fraction of the vaccinations.
Surprising spread among vaccinated people
The first report on the spread of COVID-19 in vaccinated people is based on an outbreak of 469 cases in Barnstable County, Massachusetts, where 74 percent of cases occurred among vaccinated people. Of those breakthrough cases, 79 percent were symptomatic. Over 100 of those cases were sequenced, and identified the delta variant in 90 percent of them.
Before the outbreak, the 14-day average of daily case rates in the area had plummeted to zero. But in just two weeks, the 14-day case average had shot up to 177 cases per 100,000 people.
Five COVID-19 patients were hospitalized, four of whom were fully vaccinated. None of them died.
The report shows that the highly infectious delta variant will likely continue to spread rapidly despite rising vaccination rates.
The outbreak began around the July Fourth holiday, a day President Joe Biden had hoped to mark as a day of “freedom” from the worst throes of the pandemic. But vaccinations lagged and the administration missed its target of 70 percent of American adults receiving a first shot by that date.
The report shows vaccination coverage in Massachusetts among people older than 12 was relatively high, at 69 percent, when compared to many other states.
The CDC researchers added the caveat that the data are not enough to draw conclusions about the effectiveness of the COVID-19 vaccines against the delta variant, which would require a more rigorous clinical trial.
The CDC report acknowledges that the precision of its measurement of infectiousness in vaccinated people was limited.
Monica Gandhi, a professor of medicine at the University of California, San Francisco, said more robust measurements, like examining the virus in a lab, would be needed to make the claim that vaccinated and unvaccinated people are equally infectious.
"It doesn’t make biological sense,” she said.
Gandhi said too much of a focus on the ability of vaccines to prevent transmission, rather than hospitalization and death, risks “creating incredible panic.”
The CDC said in the report that breakthrough infections will likely grow into a larger proportion of cases as vaccination rates increase.
The report follows a confidential briefing that the CDC gave to Congress Thursday in which Walensky noted the number of cases are higher than they were at this point in the summer last year and hospitalizations are at the same level — although a vaccine was not available then.
That could be driven in part by more public gatherings this year like the one in Massachusetts.
The documents shared at the confidential congressional briefing also show that vaccine efficacy has taken a hit from delta. The CDC estimates efficacy may be 75 to 85 percent against the new variant rather than 94 percent effective or more against the original COVID-19 strain.
There are 35,000 symptomatic breakthrough cases each week, the presentation shows.
Up to 15 percent of deaths in May were among vaccinated people, the presentation shows. That contrasts with previous public CDC data showing deaths occur in a tiny number of vaccinated people, just 0.0005 percent.
But that may reflect that older, immunocompromised people made up a larger proportion of vaccinations earlier in the year.
The slides presented at the briefing stated that delta is as transmissible as chickenpox and that the CDC needs to make it apparent to the public that “the war has changed.”
“I read a certain level of desperation in what the CDC is saying here,” said Paul Offit, a University of Pennsylvania vaccinologist. “I think they’re frustrated that they can’t get more people to take a vaccine. You would have thought that more than 600,000 people dying would have caught their attention.”
Offit attributes the rise in cases to the delta variant, continued resistance to vaccines and much riskier behavior by many Americans.
“It’s always been a pandemic of the vaccinated. Now it’s a pandemic of the willfully unvaccinated,” he added.