Q. ARE THE BOOSTERS SHOWING EFFICACY AGAINST OMICRON?

A.Two types of data tell us something about variant resistance to the existing Covid vaccines – laboratory test-tube data, and population epidemiologic data. Both are helpful but neither is perfect.

TEST-TUBE DATA

A paper was published in December as a pre-print (not yet peer reviewed) from scientists at the Kirby Institute in Sydney: They assessed the impact of Omicron infection on the ability of various things to neutralise the virus in the test tube. They examined (1) serum from vaccinated and/or (2) previously infected individuals, (3) concentrated human antibodies from Blood Bank plasma donors, and (4) licensed monoclonal antibody therapies.

They showed greater resistance to neutralisation of Omicron compared to Beta, Gamma and Delta across serum from both AstraZeneca and Pfizer double-dose vaccinated donors.

There was a 17 to 22-fold reduction in neutralisation titres across all donors who had a detectable neutralising antibody titre to the Omicron variant.

Of all therapeutic antibodies tested, significant neutralisation of the Omicron variant was found for Sotrovimab (GSK) and Tixagevimab (AstraZeneca), but other monoclonal antibodies were unable to neutralise Omicron in the test tube. On the basis of some modelled projections from these test tube data, they predicted that even with the 20-fold decrease in neutralisation titre, boosting with mRNA vaccines (either Pfizer or Moderna) would provide significant protection from infection with Omicron (shown in table below).

REAL-WORLD DATA OF VACCINE EFFECTIVENESS IN ACTUAL POPULATIONS AFFECTED BY OMICRON

Omicron variant:

From a recent UK government study, among those who had received two doses of AstraZeneca vaccine, there was no protective effect of vaccination against symptomatic disease with Omicron from 15 weeks after the second dose.

For those who had received two doses of Pfizer, vaccine effectiveness was 88 per cent (2-9 weeks after dose two), falling to 48 per cent at 10-14 weeks post dose two and further to 34-37 per cent from 15 weeks post dose two.

Among those who received AstraZeneca as the primary course, from two weeks after a Pfizer booster dose, vaccine effectiveness rose to 71 per cent. Vaccine effectiveness increased to 75 per cent after the booster among those who had received two doses of Pfizer as the primary course. Delta variant:

Vaccine effectiveness drops from 76 per cent 2-9 weeks after dose two, to 42 per cent at 25+ weeks after dose two with an AstraZeneca primary course. Effectiveness increases to 94 per cent two weeks after a Pfizer booster.

With a Pfizer primary course, effectiveness drops from 88 per cent 2-9 weeks after dose two to 63 per cent 25+ weeks after dose two, increasing to 93 per cent two weeks after the booster.