Temporary protection against omicron needs mRNA booster: limited data for those at highest risk

Simon Collins, HIV i-Base

A UK study published in the NEJM reports waning protection from using two primary doses of the Oxford and Pfizer vaccines, but that booster with mRNA vaccines six months later can increase protection for a limited time.

The study used a case-control design including more than 880,000 people with omicron, 200,000 with delta and 1,500,000 negatively tested individuals (from November 2021 to January 2022).

At all timepoints, and for all vaccine combinations, vaccine efficacy was higher against delta than omicron.

By 20 weeks. primary doses of the Oxford vaccine was estimated to no longer provide protection against symptomatic disease from Omicron, but this increased to 62% 2-4 weeks after a booster with Pfizer. This protection waned to 39% after ten or more weeks. Responses to a Moderna booster were higher at 70% and 60% for the same timepoints, respectively.

The efficacy of primary doses of the Pfizer vaccine dropped to 8% by week 25, increasing to 67% after a Pfizer booster, and dropping to 45% ten or more weeks later. Responses to a Moderna booster were similarly higher than a Pfizer boost, at 74% and 64% for the same timepoints, respectively.

However, as with phase 3 studies, the majority of participants were not at high risk: only 7% were older than 65 and only 12% were from Black or south Asian populations, By vaccine priority group, roughly 6% were health workers, 5% were extremely clinically vulnerable, 18% were at risk and less than 1% were severely immunocomprimised.


These data support the importance of using mRNA vaccines as booster doses in the UK and show the need for a similar response globally to protect against the omicron variant.

Longer follow-up is needed to understand the duration of protection from the booster dose and future strategies.

As with most studies, this paper included very limited data on people who are severely immunocompromised.


Andrews N et al. Covid-19 Vaccine Effectiveness against the Omicron (B.1.1.529) Variant. NEJM. DOI: 10.1056/NEJMoa2119451. (2 March 2022).

First published 3 March 2022.


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