The CDC’s new mask guidance is based on confusing information that has been misrepresented in the press.

First, everyone needs to get a COVID vaccine. Unvaccinated people are at great risk for contracting COVID-19 and getting sick because the Delta variant appears to be more infectious. An unvaccinated person who gets COVID-19 Delta variant will likely spread it to many people.

Essentially, each unvaccinated individual infected with the Delta variant coronavirus is a super spreader event unto themselves. These facts are incontrovertible at this point in the pandemic.

In contrast, earlier this week the CDC reversed itself and suggested that individuals who have received COVID-19 vaccines should resume masking themselves when indoors especially in areas where there is a high level of COVID-19 spread. I was asked by readers to comment on this recommendation and admit I too was confused by the reversal of mask guidelines for the vaccinated.

The CDC gave no scientific data behind the reversal and seemed to indicate that there was no reason to believe the vaccines were not effective in preventing COVID-19, even that caused by the Delta variant. Despite this, the news agencies touted “leaked reports” and “secret slide decks” that raised concerns about the COVID-19 vaccines.

The data the CDC used in making this decision was finally published yesterday (Friday) in Morbidity and Mortality Weekly Report (MMWR). To my surprise, there was no information that suggested the vaccines were not working and no data specifically implicating vaccinated individuals as spreading COVID-19 (see figure below).

,CDC slide from “mask reversal” deck showing marked reductions in COVID-19 infections, hospitalizations and deaths in people who are vaccinated.

What was presented in the MMWR publication is a jumbled mess of contradictory facts about a unique super spreader event that has questionable application to other outbreaks across the country.

The MMWR report is focused on a Delta variant outbreak of COVID-19 in Provincetown, Massachusetts from July 3-7, 2021. Before the outbreak, 69% of local residents were fully vaccinated, and there was a county wide 14-day average of zero COVID-19 cases per 100,000 individuals on July 3. Thus, the pandemic was firmly in control in Provincetown before the outbreak.

The outbreak was presumably triggered by several large public gatherings that attracted individuals from throughout the U.S. Provincetown has a thriving LGBT culture, and while it is not stated in the report, many of these events appear to be parties and concerts specifically targeted at the male segment of this community. This was reflected in the individuals who became infected in the outbreak, who were 85% male. Many of the events were mixers, where people were encouraged to mingle in close crowds.

The Provencetown outbreak in Barnstable County Massachusettes. Strangely, there were more infections in vaccinated (light blue) than unvaccinated (dark blue) individual.

There are several confusing aspects of this outbreak. 74% of local residents who were infected were completely vaccinated. This is higher than the 69% of local residents who are fully vaccinated, suggesting there was preferential infection in vaccinated individuals–something that has never been seen elsewhere. The vaccinated individuals who were infected with COVID-19 were also predominantly symptomatic, which differs from other reports.

Finally, four of the five hospitalized individuals were fully vaccinated, which is very different from the national figures (shown above) where 96% of hospitalizations are in unvaccinated individuals. The report states that there were “underlying medical conditions” in half of these individuals, but did not otherwise elaborate. No one died from COVID-19.

A few caveats here that may partially explain the unusual numbers. Sixteen percent of the “fully vaccinated” individuals received the Johnson and Johnson vaccine, a much higher percentage than nationwide. This single dose vaccine is significantly less effective than the two dose mRNA vaccines from Pfizer and Moderna. In addition, 30 of the individuals had verified HIV infection, but all appeared to have stable disease on antiretroviral therapy and none of these individuals was hospitalized for their COVID-19.

The most contentious “finding” of the Provincetown outbreak was that vaccinated individuals had “as much COVID-19 virus in their nose as un-vaccinated individuals.” This is what triggered the change in the masking recommendations. There is a problem with this statement, however, since the virus was quantified by PCR amplification of nasal swab and not by viral culture.

PCR cannot distinguish live from dead virus, and the vaccinated individuals might have killed the virus in their nose given their immunity. PCR also is highly variable because of its amplification of the RNA signal. Because both vaccinated and unvaccinated individuals required 33-million-fold amplification of their samples in PCR to detect virus it is hard to understand how much virus was actually present! 

Contrast this outbreak with two new reports from Israel. First, the scientists who suggested that the Pfizer vaccine was much less effective against the delta variant retracted their statements as they had not accounted for variations in non-vaccination rates in different parts of the country. Along with the CDC’s data, this suggests that full doses of the mRNA vaccines continue to provide strong protection against the delta variant.

More importantly, a New England Journal of Medicine article examining breakthrough cases of COVID-19 in immunized health care workers showed that these breakthroughs were rare (39 breakthrough infections in 11,453 individuals, 1,497 of whom were tested for infection), were predominantly asymptomatic, and that although these individuals shed some virus detectable by PCR, they had much less virus than expected especially when looking for antigen (live virus).

While this study predominantly involved the alpha variant COVID-19, it reinforced the conventional thinking about vaccine breakthrough infections and shows the power of the Pfizer COVID-19 vaccine.

The bottom line is that the Delta variant is highly infectious, and anyone without a vaccine should have to wear a mask all the time since, if they get infected, they can spread the infection to many individuals. This includes rare numbers of vaccinated individuals. Despite this, the vaccines still appeared to be very effective in preventing COVID-19 infection and illness even with the delta variant. Therefore, the real risk to vaccinated individuals remains the unvaccinated.

Published by jbakerjrblog

Immunologist, former Army MD, former head of allergy and clinical immunology at University of Michigan, vaccine developer and opinionated guy.

10 thoughts on “The CDC’s new mask guidance is based on confusing information that has been misrepresented in the press.

  1. Dr. Baker – are you aware of any studies investigating if it is okay/advantageous for a J&J shot recipient to get an mRNA shot to increase immunity? My husband had the J&J and I am so concerned that he isn’t as protected as I am (I had the Moderna shot). We followed the guidelines of getting him the first available shot, but I now wish we had waited until the Pfizer/Moderna had become available! Thank you for your knowledge!


    1. No studies with JnJ but there are studies suggesting this works with the AstraZeneca vaccine. Since they are similar I would guess it will work with the JnJ vaccine.


  2. Can you comment on need for people who have had Covid to get vaccinated? Yes or no? No mention if this % of population when we bin people into the vac/unvac groups. Thank you.


  3. Here’s something I read in an otherwise rational and well-written post today. I would be very grateful to hear what you have to say about it. “i am particularly alerted to the possible negatives from the effects of ADE (antibody-dependent enhancement), which scientifically explains how variants can emerge from the mRNA vaccine protocol itself and where data showing the added viral load in those previously vaccinated who now contract covid is much heavier than previously discussed.”

    Many thanks for your expert and very lucid posts on this subject.

    Peter Grudin

    Liked by 1 person

  4. Dr thank you so much for this clarification. Your commentary has been a beacon throughout all the misinformation. I do have a question – my husband read an article saying that overtime the Pfizer vaccine is losing its potency and at say at 6 month after vaccine we are only at about 60 percent efficacy or less. Is this true? And since a booster is not available yet would there be benefit to get revaccinated or maybe just get one to boost up? And then what about the Moderna – there was no mention of it’s longevity. Please advise. Thx!


    1. See today’s post, as according to the Israel health folks Pfizer is 80+% effective against infection, but shows no loss of protection for hospitalization or death from delta at six months. They are recommending boosters only in those 65+ or with serious health and immune problems. No data on Moderna (Israel used only Pfizer) but you can assume it is at least as good as Pfizer.


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