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Although Omicron sub-variants have evolved to evade antibody responses from the primary Covid vaccine series, a new laboratory study suggests existing booster doses may elicit sufficient immune protection against severe Covid-19 disease from the highly transmissible strain.

Omicron has greatly diverged from the ancestral SARS-CoV-2 strain. It consists of several sublineages, including BA.5, which is predicted to soon dominate globally by replacing other variants. The protective immunity generated by the primary vaccine series or prior infection has been severely blunted by Omicron sub-variants.

But a booster dose, regardless of the type of Covid vaccine, brings neutralising antibodies against all the Omicron sub-variants to appreciable levels, according to the findings published in the journal Science.

Researchers from the University of Washington first examined the functional impact of the mutations in the Omicron sub-variants spike proteins.

They found that the ability of the Omicron BA.5 spike to bind with its host receptor (ACE2) was more than 6 times stronger than the ancestral SARS-CoV-2 strain.

The researchers then evaluated, in human plasma samples, the neutralising activity, elicited by vaccines or by prior infection, against the various Omicron sub-variants. Some of the samples came from individuals who had Covid very early on in the pandemic before vaccines were available.

Only 5 of these 24 early-pandemic individuals had detectable neutralising activity in their plasma against any of the four Omicron sublineages tested. Even then, their response was very weak.

The team also evaluated the subvariant neutralising antibodies elicited by vaccines produced by Moderna, Pfizer, Novavax, Janssen, AstraZeneca, Sinopharm, and Sputnik V vaccines. All vaccine primary series consisted of two doses, except for Janssen's vaccine which consisted of a single dose.

"Overall, the data underscores the magnitude of evasion of polyclonal plasma neutralising antibody responses for Omicron sublineages," the researchers said.

They added that there was "a subtle but consistently more marked effect for BA.1 and even more so for BA.4/5 compared to BA.2 and BA.2.12.1".

However, "the marked improvement in plasma neutralising activity for subjects that received a booster dose over those that did not highlights the importance of vaccine boosters for eliciting potent neutralising antibody responses against Omicron sublineages".

Thus, despite the high degree of Omicron subvariant immune escape, the team added booster doses with currently available vaccines will likely offer strong protection against severe disease.