Some Notes on COVID Vaccines and the Delta Variant

I’ve spent some time over the last few days trying to puzzle out the available information on vaccine efficacy, the Delta variant and what if anything we can see about the future from what’s happening today in a few test case countries around the world.

As we’ve discussed Israel remains the most vaccinated and most effectively vaccinated country in the world. It’s current situation illuminates several aspects of the COVID present and future. (They use the Pfizer vaccine almost exclusively.) The country was basically done with COVID a couple weeks ago and had ditched basically all mitigations and restrictions. Then there was a new outbreak based largely on the Delta variant. But this is a case where ‘outbreak’ is a highly fluid and without context almost meaningless word. For the first couple weeks of June daily infections were in the single digits. Then they jumped to about 100 a day and then 200 a day. That’s a very large jump in percentage terms but for a country of around 9 million people that remains a very, very low number. But in response Israel reinstated its indoor mask mandates and is now rushing to vaccinate the country’s 12-16 year olds. More than 80% of adults in the country are vaccinated. But the country has been more tentative about vaccinating this early teenage cohort because of concerns about extremely rare but now tentatively confirmed cases of myocarditis in young men.

Over the weekend those numbers appear to have stabilized and begun to ebb down. This lends credence to the suggestion that the outbreaks are tied specifically to a couple super-spreader events at schools in northern Israel and one day at Ben Gurion Airport when large crowds led officials to allow visitors to skip testing. It suggests that even with the hyper-spreading Delta virus the country’s level of vaccination is keeping spread very low and low enough to allow contact tracing and ad hoc quarantines to snuff out small outbreaks.

Critically, despite the small uptick in cases, the levels of active disease and mortality have remained stable or in fact continued to fall.

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The number that jumped out at me when I first heard this news out of Israel was that about a third or more of the new infections were among people who are vaccinated. Health officials have said either one-third or as much as one-half in different contexts. For a vaccine that has an efficacy over 90% against the original COVID, more than a third sounds bad. But people who have a more intuitive understanding of statistics remind me that when a pretty high percentage of the population is vaccinated that percentage is going to need to be pretty high. Indeed with more than 80% of the population over age 16 vaccinated it’s probably the case that non-vaccinated persons will have a hard time running into to contagious carriers until the virus makes some dent in the vaccinated population. And other very recent studies suggest that the Pfizer vaccines and to a slightly lesser extent the Astra-Zeneca vaccine remain highly effective against the Delta variant, and especially severe cases of it.

There is clear a non-trivial delta (small-d) between the efficacy against old-fashioned COVID and the Delta variant. It doesn’t seem large. Probably not large enough to have much impact on you personally. But precisely how large it is remains fuzzy. Notably, one public health official in Israel noted that we don’t know if the Delta variant could be making substantial progress against the vaccine when it comes to asymptomatic cases. It’s much more challenging to measure that. We’ve had time to do it with COVID in general. The Delta variant just hasn’t been ubiquitous for long enough to know.

I’ve also been discussing this with TPM Reader XX who does the kind of computational statistical analyses that allows you to infer the efficacy of a vaccine based on case rates, vaccination density and so forth. As he told me, the data coming out of Israel isn’t detailed enough yet to really do this with any precision. Another issue XX mentioned to me is that a lot of the reporting – not only public, non-technical reporting but even discussion in the technical literature – doesn’t always use a consistent definition of what efficacy means. XX mentioned Dr. Monica Gandhi as a good Twitter follow (and expert in general) on this topic.

Adding new drama to the story, the WHO just put out new guidance which can easily be read as instructing people who’ve been vaccinated to continue doing most or all of the mitigations we’ve been doing for the last year-plus because of the Delta virus: masking, hand-washing, avoiding poorly ventilated spaces, etc. That threw a lot of people for a loop, in different directions depending on your sensibilities. I think it illustrates more than anything else that it’s really, really hard to provide 8 billion people with guidance under all the radically different settings they find themselves in.

There seems to be some question whether WHO is concerned that the robust immunity to the Delta variant provided by the best vaccines may still along spread through a still unrecognized level of asymptomatic infections. A simpler explanation could be an effort to exhort people to some basic commonsense. If you’re in a region with high transmission of the Delta variant it probably makes sense to wear a mask in closed indoor spaces if you’re around a lot of people even if you’re vaccinated.

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