Posts Misinterpret CDC’s Provincetown COVID-19 Outbreak Report

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SciCheck Digest

Social media posts are misinterpreting the results of a Centers for Disease Control and Prevention report, which found 74% of people in a COVID-19 outbreak were vaccinated, to argue against immunization. But experts say the headline-grabbing statistic is misleading without more context — and doesn’t mean that the vaccines don’t work.

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A statistic from a CDC report about a COVID-19 outbreak in July is fueling false and misleading claims on social media about how well the Food and Drug Administration-authorized vaccines protect against the disease, which is caused by the novel coronavirus.

Clinical trials and studies of the vaccines under real-world conditions show that the immunizations are highly effective in preventing severe disease and death, even against the highly contagious delta variant — although they may work less well against infection and symptomatic disease with delta, as we’ve written. The delta variant now accounts for around 93% of COVID-19 cases in the U.S., according to estimates from the CDC.

The statistic in question comes from a paper published on July 30 in the agency’s Morbidity and Mortality Weekly Report, which documents an outbreak of COVID-19 in Barnstable County, Massachusetts — elsewhere specified as Provincetown — that primarily occurred in vaccinated people, following large public events in the first half of the month.

According to the report, of the 469 people included in the study who were in the area between July 3 and July 17 and tested positive for the coronavirus, 74% were fully vaccinated. A total of five people were hospitalized, four of them vaccinated, and there were no deaths. 90% of the subset of people who had sequencing performed on their samples were infected with the delta variant.

Although not mentioned in the report, the outbreak overlapped with July Fourth weekend and “Bear Week,” Provincetown’s annual gathering of gay men; 85% of the identified infections were in males. In the summer, the town’s population swells to approximately 60,000 people.

The report noted that at the time of diagnostic testing, the amount of virus appeared to be about the same in vaccinated and unvaccinated people, a finding that the CDC cited in its decision on July 27 to recommend masks for everyone, regardless of vaccination status, in indoor public spaces in areas with “substantial” or “high” transmission.

But the 74% figure, while correct, can be misleading without the proper context, experts say, because as vaccination rates increase, it’s entirely expected for a larger and larger proportion of people who are infected to be vaccinated. It doesn’t mean the vaccines don’t work.

“The problem is we are only looking at those who got infected, not at everyone in the area who was at risk of being infected,” Matthew Fox, an epidemiologist at Boston University School of Public Health, told us in an email. “Provincetown is an area with some of the highest vaccination rates in the country, so if the vaccine was not working, you’d expect the % vaccinated among the infected to be even higher than 75%, but we can’t say that for sure yet because we don’t know the denominators, we’d need more data. That said, all the carefully done studies to date that have included the denominators have shown the vaccine to be highly effective, even outside of the trials.”

The paper says as much, writing that “data from this report are insufficient to draw conclusions about the effectiveness of COVID-19 vaccines against SARS-CoV-2, including the Delta variant, during this outbreak” and noting that as vaccination coverage increases, “vaccinated persons are likely to represent a larger proportion of COVID-19 cases.”

Numerous posts on Instagram and Facebook have nevertheless seized upon the 74% figure, often using screenshots of news headlines, without the proper context or with additional commentary to give the misleading impression that this is evidence that the vaccines don’t work, that vaccinated people are just as likely or even more likely to spread the virus as those who are unvaccinated, or that something nefarious is afoot.

“We tried to tell y’all do not take their Experimental Vaccines,” reads one Instagram post, sharing a headline from CNBC.

Another, sharing a similar headline, falsely implies that vaccinated people are more likely to spread the coronavirus than the unvaccinated. “REALITY CHECK!! The vaccinated ARE the super spreaders,” it says.

“CDC study shows 74% of people infected in MA Covid outbreak were fully vaccinated debunking the ‘pandemic of the unvaccinated’ narrative,” reads an Instagram post, which is accompanied by a video of CDC Director Dr. Rochelle Walensky and President Joe Biden.

Another claims Walensky admitted the vaccines “are now failing, and vaccinated people may now carry higher viral loads than unvaccinated people” and that “the people that got the Vaccine are the biggest carriers.”

The CDC has used the MMWR report as justification for its change in mask guidance, interpreting the similar amounts of virus at the time of testing among the vaccinated and unvaccinated people in the outbreak as a sign that infected vaccinated people can transmit the virus. But as we’ll explain, that does not mean that the vaccinated and unvaccinated are equally likely to infect others.

And while some vaccinated people have become infected with the coronavirus and have likely passed it on to others, the larger concern is still about unvaccinated people, who are far more likely to become infected, fall seriously ill and spread the virus.

74% Statements Lack Context

Posts and headlines that report that 74% of the COVID-19 cases in the Provincetown cluster were in vaccinated people are prone to misinterpretation because on the surface, that fact can seem surprising if the vaccines are supposed to work so well. 

“When a new vaccine is introduced into a population, when vaccination rates become very high, we can expect as many and sometimes more cases in the vaccinated population than the unvaccinated — even when the vaccine is doing its job and protecting people at a high rate — because the denominator of number of people vaccinated is so large,” Boston University’s Fox explained in a blog post. “Epidemiologists who study vaccinations are very aware of this phenomenon and yet, even though we expect it, it is hard to communicate the accurate and important message to people that this isn’t a sign of vaccine failure.”

As we have written, because no vaccine is 100% effective, you do expect some cases in vaccinated people — and even with a very effective vaccine, the proportion of cases among the immunized can be high if much of the population has been vaccinated. 

To understand this, it’s helpful to consider an extreme example in which everyone in a given place has been immunized with a highly effective vaccine. “If you had a small outbreak in a town that was 100% vaccinated, 100% of the cases would be vaccinated,” William P. Hanage, an epidemiologist at Harvard’s T.H. Chan School of Public Health, told us, “but it wouldn’t mean the vaccines don’t work!”

Foot traffic along Commercial street in Provincetown, MA on July 20, 2021. An outbreak of COVID-19 occurred in the town in the first half of July following a series of large public events. (Photo by Barry Chin/The Boston Globe via Getty Images)

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.”

Editor’s note: SciCheck’s COVID-19/Vaccination Project is made possible by a grant from the Robert Wood Johnson Foundation. The foundation has no control over our editorial decisions, and the views expressed in our articles do not necessarily reflect the views of the foundation. The goal of the project is to increase exposure to accurate information about COVID-19 and vaccines, while decreasing the impact of misinformation.


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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.

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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.

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