Posts Misinterpret CDC’s Provincetown COVID-19 Outbreak Report – FactCheck.org

While it’s not known how many visitors to Provincetown were vaccinated, anecdotally many of them were, and data from the state show that prior to the outbreak, virtually everyone in the town had received at least one dose of a vaccine, with 85% or more of each age group fully vaccinated.

“The 74% needs to be put in the context that a very high proportion of the people exposed were vaccinated,” said Hanage. “It suggests that in the absence of vaccination the outbreak would have been much larger.”

Fox has explained in a separate blog post that the information you’d need to know how well the vaccines performed is how many vaccinated and unvaccinated people were in the area at the time.

“We could then calculate the percent of people infected in the vaccinated group and the percent of people infected in the unvaccinated group and compare them to see how well the vaccine was or was not working,” he wrote.

But that information isn’t available in this case, which is why the report acknowledges that it can’t comment on vaccine effectiveness.

The studies that do exist on vaccine effectiveness, however, indicate that the FDA-authorized vaccines may be a bit less effective in preventing infection and symptomatic disease against delta compared with previous versions of the virus, but they remain highly effective against severe disease and death. Available data in the U.S. are consistent with those findings, and thus far do not indicate any major problems.

As of July 26, for example, the CDC had received 6,587 reports of patients with breakthrough infections who died or were hospitalized, out of more than 163 million vaccinated people. Even those figures may be misleadingly high, as around a quarter of the hospitalizations and deaths were reported as “asymptomatic or not related to COVID-19.”

Data from individual states — collected and reported by the Kaiser Family Foundation, because the CDC began only tracking breakthrough infections involving hospitalization and death in May — similarly suggest breakthrough cases of any kind are rare.

“The rate of breakthrough cases reported among those fully vaccinated is well below 1% in all reporting states,” KFF found in its July 30 analysis. (It’s important to remember that these numbers, too, will increase over time, even if the vaccines continue to work — the part that matters is to see how this compares to the percentage of the unvaccinated who get sick or die.)

While many have viewed the Provincetown outbreak as concerning — and it does show that vaccinated people can become infected and fall ill — epidemiologists also see it as evidence of the vaccines’ success.

“In general if I had been told last summer that Bear Week would go ahead in 2021 with minimal interventions, and lead to an outbreak of 900+ but that the upshot would be 7 hospitalizations and no deaths, I would have been amazed,” said Harvard’s Hanage, referring to case totals beyond those reported in the MMWR study. “Even more so if I’d been told about Delta.”

Vaccinated People Less Likely to Spread Virus Than Unvaccinated

Much of the alarm about the Provincetown outbreak has focused on PCR test data among a subset of the outbreak participants suggesting that infected vaccinated individuals have similar viral loads to those who are unvaccinated. This was indicated by similar so-called cycle threshold values from the tests, which can serve as a rough proxy for how much viral RNA was in a person’s sample. Lower values indicate fewer cycles had to be run to detect the coronavirus and therefore that there was more RNA in a specimen.

The CDC has noted this finding in explaining its rationale for recommending that vaccinated people also wear masks in areas with “substantial” or “high” transmission when indoors in public places. (See SciCheck’s story, “A Guide to the CDC’s Updated Mask Recommendations.”)

Contrary to claims online, however, this does not mean that vaccinated people are equally or more likely to spread the coronavirus than unvaccinated people. Even if it’s true that vaccinated people harbor similar amounts of infectious virus — and immunologists have reason to be skeptical of this — vaccinated people would still be significantly less likely to transmit the virus because they’re less likely to be infected in the first place, as we’ve noted before.

Some experts think the CDC is over-interpreting the MMWR report’s results, although they say the agency is still right to recommend masks in addition to vaccines. That’s because the diagnostic test data in the report are unlikely to fully reflect how infectious someone is, particularly in vaccinated people.

For one, these sorts of PCR tests are good at identifying viral RNA, but they can’t tell whether that genetic material is in an intact, infectious virus particle or not. That becomes especially relevant for vaccinated people, Deepta Bhattacharya, an immunologist at the University of Arizona College of Medicine, said.

“Antibodies from a vaccinated person can coat the released virus and keep it from infecting other cells,” he told us. “And T cells can kill infected cells, releasing viral genetic material but not infectious particles.”

Second, the tests are only looking for RNA present in the nose and throat, not the lungs — even though vaccines are likely to have more of an impact there, according to previous research.

“Though it isn’t entirely clear how much of transmission comes from the lungs vs. the nose and throat,” Bhattacharya said in an email, “it is almost certainly some.” That would also suggest a vaccinated person with a similar cycle threshold as an unvaccinated person would be less infectious.

Vaccinated people also likely aren’t infected as long, since their immune systems are quicker to respond to the virus, which would also make them less likely to infect as many people as an unimmunized person.

Bhattacharya pointed to an unpublished study from Singapore, posted to the preprint server medRxiv on July 31, that found viral loads were similar in vaccinated and unvaccinated people to start, but decreased more quickly in vaccinated people.

As University of Pennsylvania infectious disease fellow Dr. Aaron Richterman noted on Twitter, along with the faster decline in detectable viral RNA, the study found fewer symptoms in the vaccinated people. “This will have [a] substantial effect on transmission potential,” he wrote.

Preliminary results from Israel, too, Bhattacharya said, suggest that most people with breakthrough infections do not pass on the virus to others.

“[T]he CDC does have justification to recommend stronger measures to contain Delta,” he said, since it’s clear that the variant is more contagious and infections with delta appear to reach peak virus levels faster than other variants, raising the risk of transmission from a breakthrough infection. “But the implications that vaccinated breakthrough infections are just as contagious as infections in unvaccinated individuals is premature at best, and perhaps very wrong at worst.”

Angela Rasmussen, a virologist at the Vaccine and Infectious Disease Organization at the University of Saskatchewan, agreed that the CDC’s guidance to mask up is still sound, even if it’s far from clear from the Provincetown outbreak report whether the transmission potential of infected vaccinated people is really on par with the unvaccinated.

“It’s the smart, cautious thing to assume the worst and recommend that people mask regardless of vaccination status,” she said in a Twitter thread. “What we should NOT do, however, is assume that this means the vaccines don’t work.”

“If enough people are vaccinated AND taking precautions to reduce exposure,” she added, “even delta will hit too many dead ends to continue spreading in the population.”

Sources

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“Employment Opportunities.” Town of Provincetown website. Accessed 6 Aug 2021.

Fox, Matthew. Professor of Epidemiology, Boston University School of Public Health. Emails to FactCheck.org. 3 Aug 2021.

Fox, Matthew. “We are seeing more cases of COVID-19 in people who got vaccinated. Here’s why that’s not cause for concern.” Medium. 11 Jul 2021.

Hanage, William P. Associate Professor of Epidemiology, Harvard T.H. Chan School of Public Health. Email to FactCheck.org. 3 Aug 2021.

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​​Morse, Alex (@AlexBMorse). “The vaccines are working. Of the 900 cases related to the Provincetown cluster, there have been no deaths, 7 hospitalizations, and the symptoms are largely mild. Our positivity peaked at 15% on 7/15 and was only 4.8% yesterday. The outbreak is contained and Provincetown is safe.” Twitter. 30 Jul 2021.

Fichera, Angelo. “Viral Posts Distort WHO Guidance on COVID-19 Tests.” FactCheck.org. 29 Jan 2021.

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Richterman, Aaron (@AaronRichterman). “The question at hand: what is the relative transmission potential of a vaccinated person who becomes infected with delta? This ????new report from Singapore is much more informative on this question than the CT data released so far from Ptown and Wisconsin.” Twitter. 31 Jul 2021.

“80% of vaccinated COVID carriers didn’t infect anyone in public spaces — report.” Times of Israel. 25 Jul 2021.

Rasmussen, Angela (@angie_rasmussen). “Lots of understandable concern about the data that spurred the new masking guidance from @CDCgov, so let’s take a minute to discuss what this data tells us and, importantly, what it DOES NOT tell us.” Twitter. 31 Jul 2021.