We observed that Omicron infected the ACE2-expressing cells in a concentration dependent manner but did not infect the parental H1299 cells, indicating that ACE2 is required for Omicron entry (Fig. 1A). We then tested the ability of plasma from BNT162b2 vaccinated study participants to neutralize Omicron versus ancestral D614G virus in a live virus neutralization assay. We tested 14 plasma samples from 12 participants (Table S1), with 6 having no previous record of SARS-CoV-2 infection nor detectable nucleocapsid antibodies indicative of previous infection. For two of these participants, we used samples from two timepoints. The remaining 6 participants had a record of previous infection in the first SARS-CoV-2 infection wave in South Africa where infection was with ancestral D614G virus (Table S1). Geometric mean titer (GMT) FRNT50 (inverse of the plasma dilution required for 50% reduction in infection foci number) was 1321 for D614G. These samples therefore had very strong neutralization of D614G virus, consistent with sampling soon after vaccination. GMT FRNT50 for the same samples was 32 for Omicron, a 41-fold decline (Fig 1B). However, the escape was incomplete, with 5 of the participants, all previously infected, showing relatively high neutralization titers with Omicron.

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It makes sense when the vaccines were developed for the Alpha variant and now we're on Omicron, and they are two very different beasts. It would be interesting to know how much they actually have in common. I heard Omicron described as almost a different disease to the first Sars Covid 2.