NYMag | “The message that breakthrough cases are exceedingly rare and that you don’t have to worry about them if you’re vaccinated — that this is only an epidemic of the unvaccinated — that message is falling flat,” Harvard epidemiologist Michael Mina told me in the long interview that follows below. “If this was still Alpha, sure. But with Delta, plenty of people are getting sick. Plenty of transmission is going on. And my personal opinion is that the whole notion of herd immunity from two vaccine shots is flying out the window very quickly with this new variant.”
“We’re seeing a lot more spread in vaccinated people,” agreed Scripps’s Eric Topol, who estimated that the vaccines’ efficacy against symptomatic transmission, which he estimated to be 90 percent or above for the wild-type strain and all previous variants, had fallen to about 60 percent for Delta. “That’s a big drop.” Later, he suggested it might have fallen to 50 percent, and that new data about to be published in the U.S. would suggest an even lower rate. On Wednesday, a large pre-print study published by the Mayo clinic suggested the efficacy against infection had fallen as far as 42 percent.
“The breakthrough problem is much more concerning than what our public officials have transmitted,” Topol continued. “We have no good tracking. But every indicator I have suggests that there’s a lot more under the radar than is being told to the public so far, which is unfortunate.” The result, he said, was a widening gap between the messaging from public-health authorities and the meaning of the data emerging in real time. “I think the problem we have is people — whether it’s the CDC or the people that are doing the briefings — their big concern is, they just want to get vaccinations up. And they don’t want to punch any holes in the story about vaccines. But we can handle the truth. And that’s what we should be getting.”
The central distortion reflected in the Kaiser report — and echoed by communicators elsewhere, including in the Times — is the result of a basic error of comparison, one that should have been obvious to anyone familiar with the shape of the pandemic. Almost all of these calculations about the share of breakthrough cases have been made using year-to-date 2021 data, which include several months before mass vaccination (when by definition vanishingly few breakthrough cases could have occurred) during which time the vast majority of the year’s total cases and deaths took place (during the winter surge).This is a corollary to the reassuring principle you might’ve heard, over the last few weeks, that as vaccination levels grow we would expect the percentage of vaccinated cases will, too — the implication being that we shouldn’t worry too much over panicked headlines about the relative share of vaccinated cases in a state or ICU but instead focus on the absolute number of those cases in making a judgment about vaccine protection across a population. This is true. But it also means that when vaccination levels were very low, there were inevitably very few breakthrough cases, too. That means that to calculate a prevalence ratio for cases or deaths using the full year’s data requires you to effectively divide a numerator of four months of data by a denominator of seven months of data. And because those first few brutal months of the year were exceptional ones that do not reflect anything like the present state of vaccination or the disease, they throw off the ratios even further. Two-thirds of 2021 cases and 80 percent of deaths came before April 1, when only 15 percent of the country was fully vaccinated, which means calculating year-to-date ratios means possibly underestimating the prevalence of breakthrough cases by a factor of three and breakthrough deaths by a factor of five. And if the ratios are calculated using data sets that end before the Delta surge, as many have been, that adds an additional distortion, since both breakthrough cases and severe illness among the vaccinated appear to be significantly more common with this variant than with previous ones.
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