Delta breakthrough cases: rarely deadly but equally contagious.

How this breaking news changes the landscape.

During a briefing yesterday, Dr. Rochelle Walensky, the director of the Centers for Disease Control and Prevention, said something that grabbed my attention. While discussing the Delta variant, she said that when “we examine the rarer breakthrough infections [i.e. infections despite vaccination] and we look at the amount of virus in those people, it is pretty similar to the amount of virus in unvaccinated people.” This strongly implies, though does not prove, that when Delta manages to infect a vaccinated person and cause symptomatic Covid-19, that person remains highly protected from serious disease, but on average they may be just as likely to spread the virus forward as unvaccinated infected people. In scientific terms, the viral load, a very good proxy for contagiousness, among unvaccinated and breakthrough delta infections appear comparable.

The viral load, a very good proxy for contagiousness, among unvaccinated and breakthrough Delta infections appear comparable.

This is breaking and important news. It’s the reason why the CDC has now altered its mask guidance in areas with “substantial or high” levels of transmission, advising indoor masking in public even for vaccinated people in these places. Given the threshold of 50 new cases per 100,000 people in a 7-day period, the new guidance applies to large swaths of the country now.

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Many had already begun to suspect that Delta could be contagious in a few breakthrough cases. But until Dr. Walensky’s comments today—which corroborated anecdotes from the NBA implying similar findings—I did not have any inkling of how rare or common this might be.

Things had already begun to change for me last week when two acquaintances tested positive for coronavirus despite receiving two doses of an mRNA vaccine this spring. Neither of them particularly enjoyed being sick, but both were at home, rather than hospitalized. What probably sounded scary to some—breakthrough infections—to me actually looked to be reassuring examples of the vaccines doing their job by preventing critical illness and death. But one of the two cases took me aback. She had tested positive via a rapid antigen test. That piece of information had important implications about her contagiousness.

To understand why, let's refresh on test types. Most tests administered are PCR tests. PCR tests check for the genetic material of SARS-CoV-2, the virus that causes Covid-19. A positive result means the person was infected “sometime recently,” but does not indicate contagiousness. In fact, many people remain positive on PCR tests for weeks, well after a person has won the initial fight and ceased being a danger to others. Rapid antigen tests work by detecting a particular protein on the surface of the virus. Only viable contagious virus harbors enough surface antigen to mount a positive test. A person only tests positive on a rapid test at times when they are contagious. That means my second friend, the one with the positive rapid antigen test, was vaccinated, infected, and contagious. So far, that combination is unique to the Delta variant.

She had tested positive via a rapid antigen test. That piece of information had important implications about her contagiousness.

What we learned today, both from Dr. Walensky and other well-sourced observers is that once a vaccinated person is infected with Delta (and at this point, assume all US cases are Delta), may be equally contagious as an unvaccinated person. (For my science colleagues, we are being told that the “cycle threshold” on the PCR tests, a number which indicates viral load, were in the teens and twenties, figures that imply the possibility of transmissible virus.) That said, most infections, hospitalizations, and spread are still occurring among the unvaccinated. The vaccines are still preventing most infections and most disease.

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Given that Delta now accounts for 83% of the cases in the United States, the CDC needed to adjust its strategy on masking for vaccinated people in hotspots. Was this the right call?

In my view, yes. Especially because the CDC limited its guidance to states with higher case counts. In an important first, the CDC has linked the need for a Covid-19 mitigation measure to a clearly defined public health metric. That’s good. People respond well to carrots and sticks and friendly competition; they love a good fundraiser thermometer. In the future, linking mask mandates or even closing certain things like public transit temporarily to trackable metrics like case counts (or better yet, hospitalizations per capita) would give the public a collective goal. It will also protect the oft-beleaguered CDC from being accused of “backtracking,” when what it is actually doing is trying to move as better science emerges. The CDC would do well to lay out a series of thresholds on a variety of key metrics that, if crossed, would trigger changes in its guidance.

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Many people think the CDC never should have dropped its mask mandate in May. I’m not convinced it was such a bad call. Yes, in a computer simulation, masks would be safest until everyone has been vaccinated and case counts have dropped sustainably. But in the real world, public health and government officials only have so much clout to spend. Asking people to mask for Alpha and Beta breakthroughs to prevent a minuscule amount of disease spread risked alienating them such that they would not pay attention to a moment like this, or a worse one in the future. Realistically, vaccinated people were not going to keep masking in all areas while we waited months for clinical trial data on breakthrough contagion to emerge. Indeed, by linking vaccination with the opportunity to stop masking, vaccine interest increased. If some anti-maskers were a lost cause and wouldn’t mask anyway, why not shore up vaccine rates among the willing? (The CDC denies this motive for its May guidance, but it makes sense.)

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What we need now is more data. Any data! Currently, we only know that the average viral loads (that crucial proxy for contagiousness) among unvaccinated and breakthrough Delta cases are similar. And that’s just us taking the CDC’s word for it on what we are being told is around 150 cases. That’s goodwill that public health professionals like me can extend for a few days, but not much longer. In particular, I’m eager to evaluate at least two more crucial pieces of missing information.

First, how do the viral loads among the “worst” 10% of cases compare? My personal belief is that a great deal of coronavirus spread comes from a relatively small subset of highly contagious cases. If our vaccines continue to render the most contagious cases less so, it might mean that breakthrough Delta is not really as contagious as unvaccinated cases are, even though the viral loads of the average cases are close. Second, how long are breakthrough Delta cases contagious? Some breakthrough cases might be contagious for as long as unvaccinated cases (i.e. up to 10 days in most instances), but many may not. Once we know that, we’ll be able to make more refined recommendations. For now, if you’re in a high-risk zone, vaccinated or not, indoor masking in public makes sense.

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Works referenced/further reading and resources:

The CDC’s new mask guidance, July 27, 2021: https://www.cdc.gov/coronavirus/2019-ncov/vaccines/fully-vaccinated.html

The CDC’s Covid-19 hotspot map: https://covid.cdc.gov/covid-data-tracker/#county-view

New York Times on breakthrough infections: https://www.nytimes.com/2021/07/22/health/coronavirus-breakthrough-infections-delta.html

CDC Briefing (July 27, 2021) audio: https://www.cdc.gov/media/releases/2021/t0727-covid-19-telebrief.mp3

Vaccine interest went up after the CDC’s mask guidance in May: https://www.cnn.com/2021/05/27/health/vaccination-interest-cdc-mask-guidance/index.html