Day 436: Vaxzevria to Blame for VITT

Science reports on the causal connection between AstraZeneca's COVID-19 vaccine "Vaxzevria" and a rare clotting disorder scientists have dubbed vaccine-induced immune thrombotic thrombocytopenia (VITT). VITT has affected at least 222 European vaccine recipients and killed more than 30 of them (a rate of about one in a million).

While the disorder is similar to heparin-induced thrombocytopenia (HIT), the mechanism is only partly understood. The mRNA-based vaccines (Moderna's and Pfizer's) don't appear to prompt VITT, but suspicious clotting has already been reported for another viral vector vaccine, Johnson & Johnson's.

One in a million is a very small risk, possibly even smaller than the risk of a child under 10 dying of COVID, but several European countries have reacted by restricting the vaccine to the elderly. The article offers a glimmer of hope for Vaxzevria in lowering the doseage:

In AstraZeneca's phase III trial in the United Kingdom, a small number of people accidentally received a lower dose and had fewer side effects in general; perhaps the lower dose is less likely to trigger the kind of strong inflammation that boosts PF4 antibodies as well, the researchers say. And unexpectedly, those people were slightly better protected, perhaps because high levels of inflammation can actually block the formation of antibodies, Marschalek says. “Part of the problem might be that they just overdose” the vaccine, Greinacher says.

In local news, Massachusetts' cases have been holding around a third of a percentage point for the past week. (Today's numbers are not up yet.)