We've had some interesting vaccine news in the last few days, and it's worth a closer look. A team from the NIAID, Emory, Moderna (and others)
has reported results in a primate model for an Omicron-targeted mRNA booster shot that they've been working on, and the numbers are. . .a bit surprising. Macaque monkeys were dosed twice, four weeks apart, with the standard Moderna Coronavirus vaccine, and then 41 weeks later one group of them got a booster of the same shot, while another got a booster of the new one with an Omicron variant sequence. Subsequent tests for neutralizing antibody levels, B-cell expansion, and response to a challenge with the Omicron virus itself showed that there was
no difference between the two treatments at all.
It's important to say right up front that both vaccine regimens did a strong job of protecting the test animals - strong enough that both groups of monkeys were pretty much completely protected in the lungs during the challenge study, which in its way makes comparison at that point a bit difficult (protection in the upper airway was strong, but less complete, as it is in humans). So I hope that people don't get confused as this news gets out into thinking that the Omicron-focused booster did nothing. It worked fine; it's just that it brought nothing extra compared to the regular booster.
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The authors believe that this is most likely due to the phenomenon known (catchily) as "original antigenic sin", or less rousingly, antibody imprinting. That's been seen in many immune responses to many different antigens over the years. A person's first exposure to a type of virus, for example, can have a noticeable effect on their later responses to similar ones.