Two doses of COVID-19 vaccines show reduced effectiveness against Omicron variant

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Two doses of the COVID-19 vaccines of AstraZeneca or Pfizer-BioNTech show a much greater decrease in their effectiveness against the Omircon variant of coronavirus than against previous strains, researchers from Oxford University have said on Monday. They stressed that this effectiveness was improved by a third dose of vaccine.
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Researchers from the University of Oxford have analysed the impact of the Omicron COVID-19 variant of concern on one of the immune responses generated by vaccination, compared with neutralisation against Victoria, Beta and Delta variants.

Blood samples from day-28 post second-dose were obtained from participants in the Com-COV2 study who had received a two-dose COVID-19 vaccination schedule with either AstraZeneca (AZD1222) or Pfizer (BNT162b2) vaccines. 

"There was a substantial fall in neutralisation titres in recipients of both AZD1222 and BNT162b2 primary courses, with evidence of some recipients failing to neutralise at all," the researchers found.

This will likely lead to increased breakthrough infections in previously infected or double vaccinated individuals, which could drive a further wave of infection, although there is currently no evidence of increased potential to cause severe disease, hospitalisation or death.

These results align with recently published data from UK Health Security Agency, showing reduced effectiveness of two doses of these vaccines against symptomatic disease due to the Omicron variant compared to Delta. Importantly, this effectiveness was improved by a third dose of vaccine.

"These data will help those developing vaccines, and vaccination strategies, to determine the routes to best protect their populations, and press home the message that those who are offered booster vaccination should take it," commented Professor Gavin Screaton, Head of the University’s Medical Sciences Division, and lead author of the paper.

"Whilst there is no evidence for increased risk of severe disease, or death, from the virus amongst vaccinated populations, we must remain cautious, as greater case numbers will still place a considerable burden on healthcare systems," he added.

"These data are important but are only one part of the picture. They only look at neutralising antibodies after the second dose, but do not tell us about cellular immunity, and this will also be tested using stored samples once the assays are available," highlighted Professor Matthew Snape, Professor in Paediatrics and Vaccinology at the University of Oxford and co-author.

Importantly, we have not yet assessed the impact of a “third dose” booster, which we know significantly increases antibody concentrations, and it is likely that this will lead to improved potency against the Omicron variant.

"Real-world effectiveness data has shown us that vaccines continue to protect against severe disease with previous variants of concern. The best way to protect us going forward in this pandemic is by getting vaccines in arms," said Professor Teresa Lambe, Professor in Vaccinology at the University of Oxford, and an author on the paper.

Increasing vaccine uptake among unvaccinated, and encouraging third doses, remain priority to reduce transmission levels and potential for severe disease, the researchers stressed.

Cover photo: Getty Images

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