The Centers for Disease Control and Prevention is fighting Covid-19 without a full arsenal of data that some public-health experts said it would need to persuade more people to take steps to contain the pandemic.
When CDC Director Rochelle Walensky said last week that people should wear masks again indoors in areas with “substantial” levels of Covid-19 transmission, for instance, she said evidence shows the Delta variant might be spread as easily by vaccinated people who become infected as by the unvaccinated. The CDC pointed to Provincetown, Mass., where it said large gatherings in July at bars, nightclubs and house parties led to hundreds of Covid-19 infections. More studies released over the weekend backed the CDC’s conclusion, Dr. Walensky said.
The Provincetown data contained two startling details: nearly three-quarters of infected people were fully vaccinated, the CDC said, and samples showed that the amount of virus infected people carried—or viral load—was similar between vaccinated and unvaccinated patients. The CDC concluded from the latter that vaccinated people who become infected might spread the Delta variant as readily as the unvaccinated.
The meaning of some of the viral load data has been disputed. Inside the CDC, some officials disagree with the agency’s conclusion that vaccinated people who become infected may spread the virus as readily as the unvaccinated, and argue that more testing needs to be done, including tests that measure how infectious virus particles are, according to a person familiar with the matter.
“There’s no one-to-one relationship between high viral load and infectivity, but we’re always making decisions based on imperfect data,” said Tom Frieden, who headed the CDC from 2009 to 2017.
Some scientists say that the Provincetown study isn’t reliable enough to be the primary driver of a public health policy change. The data is too recent to be independently reviewed by outside experts, and it is too small of a sample and the circumstances of the outbreak are too unique for it to be applied to other parts of the country.
The connection between the Provincetown data and the CDC’s new mask guidelines has struck some scientists as obscure. The guidance applied to areas with high transmission of the virus, which tend to be parts of the U.S. with lower vaccination rates. Vaccination rates in Provincetown were high, around 69% for eligible Massachusetts residents.
“They’re making these decisions on the basis of extremely weak and unreliable data, and at the same time not doing the necessary work to reduce uncertainty among the population,” said Vinay Prasad, a physician and professor in the Department of Epidemiology and Biostatistics at the University of California, San Francisco. “When there isn’t a lot of study data, the CDC should be conducting these studies.”
CDC spokeswoman Kristen Nordlund didn’t return messages seeking comment on any alleged weaknesses in the data.
The CDC’s challenge extends beyond the Covid-19 crisis. The agency often relies on samples to determine the extent of disease outbreaks or vaccination levels, and some public-health experts say its ability to collect comprehensive data in real-time is lacking.
Another point of consternation for some scientists is the tracking of so-called breakthrough Covid-19 infections among people who have been vaccinated.
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The CDC has said such infections are more common with Delta than previous variants, but the agency stopped tracking mild or moderate breakthrough infections that didn’t lead to hospitalization or death in April, before Delta emerged as a driver of the pandemic in the U.S.
Without that additional breakthrough data, scientists have struggled to understand how Delta behaves compared with earlier iterations of the virus, said Daniel Kuritzkes, chief of the division of infectious diseases at Brigham and Women’s Hospital in Boston.
“It then leaves unmeasured the extent of infection and extent of transmission among vaccinated people,” he said.
The Kaiser Family Foundation last week estimated there have been 74,713 breakthrough cases since January in the 25 states that still track the cases and report publicly on them. An internal CDC slide presentation last week estimated that 35,000 symptomatic breakthrough infections could be happening each week.
The CDC stopped tracking mild breakthrough cases because it was too labor intensive and because data from states and counties wasn’t reliable, said a federal official briefed on the matter. Local officials have since increased efforts to collect this data, the official said.
The official said the CDC is reviewing data from other recent outbreaks. Those studies together informed the conclusion that 35,000 symptomatic breakthrough infections could be happening each week, the person said.
Some public-health experts said confusing or ambiguous statements from officials including Dr. Walensky have compounded the challenge posed by the CDC’s incomplete data.
Dr. Walensky said at a congressional hearing in May that vaccinated people have a 5% chance of contracting symptomatic Covid-19 if they are exposed to a positive case. Vaccine experts said shots reduce the chance of symptomatic disease from Covid-19 infection after exposure to a positive case to much lower than 5%.
Last week, Dr. Walensky said in a television interview that if 20 vaccinated people who aren’t wearing masks interact with someone who is infectious, “one or two of them” could get a symptomatic breakthrough infection and transmit it to family members.
The CDC slide presentation last week described an incidence rate for symptomatic breakthrough infections of 21.4 cases per 100,000 vaccinated people, or 0.02%, similar to that implied by the Kaiser Family Foundation study.
“Not providing data transparently and honestly is eroding public trust,” said Tracy H?eg, a physician and epidemiologist who teaches at the University of California, Davis.
The CDC didn’t respond to requests for comment on the criticism.
The CDC has also published some puzzling data. According to the agency’s website, 197,845 children under 12 have received at least one shot. Vaccines aren’t authorized in the U.S. for children that young. Vaccine makers are evaluating whether the shots are safe for children in clinical trials.
A CDC spokeswoman said the tally could include children participating in clinical trials. The CDC also said birthdays could have been entered in its databases incorrectly. Moderna Inc. and Pfizer Inc. with its partner BioNTech SE are conducting clinical trials aiming to enroll some 11,000 total children in the U.S.
Pfizer isn’t aware of any vaccinations outside of its trial for children under 12, a spokeswoman said.
Cody Meissner, chief of the pediatric infectious diseases division at Tufts University School of Medicine in Boston, said it seemed unlikely that such a large number was a record-keeping glitch. “I don’t think you can say 200,000 birthdays were entered incorrectly,” he said.
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