Two Oxford vaccine doses ‘less effective against Delta variant than other strains'

29 June 2021, 00:02 | Updated: 29 June 2021, 11:01

Two doses of the Oxford-AstraZeneca coronavirus vaccine are less effective at producing antibodies against the Delta variant than against other strains, according to new research.

Laboratory findings from the Francis Crick Institute and the National Institute for Health Research (NIHR) UCLH Biomedical Research Centre compared results with those for the Pfizer-BioNTech jab.

Both vaccines induce lower levels of antibodies targeting the variant first detected in India (B.1.617.2), the data suggests.

The strain is now dominant in the UK and accounts for almost all new cases.

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While antibody levels alone do not predict vaccine effectiveness, the study confirms two doses of either vaccine are essential to boost antibodies to levels that are likely to maximise protection against severe disease and hospital admission.

Researchers found two doses of the Oxford jab generate antibody levels that are 2.5 times lower against the Delta variant than the Pfizer vaccine.

In people who had been fully vaccinated with two doses of the Oxford vaccine, nearly all participants had a quantifiable level (87% with greater than 40 titre) of neutralising antibodies against the variants previously dominant in the UK - the April 2020 strain, and the Alpha variant first detected in Kent (B.1.1.7).

But fewer people had quantifiable levels against the Beta variant first detected in South Africa (B.1.35) and Delta variant - 60% and 62% respectively.

This contrasted with the Pfizer analysis, which indicated that more than 95% of recipients had quantifiable neutralising antibody levels against the Beta and Delta variants after both doses.

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In people who had only received one Oxford jab, levels also varied according to previous infection.

Those with prior symptoms had higher levels of neutralising antibodies against all strains than those who hadn't been infected.

Dr Emma Wall, UCLH Infectious Diseases consultant and Senior Clinical Research Fellow for the Legacy study, said: "This is more evidence in support of ensuring everyone gets two doses of vaccine to protect against severe Covid-19, including reducing the gap between vaccine doses wherever supply and capacity allows.

"We can see clearly that the Delta variant poses a significant threat, but that two vaccine doses, and possibly an additional booster for at-risk groups, will be the best way to maximise protection, particularly against hospitalisations and deaths from Covid-19.

"These vaccines were designed based on the original strain first detected in China in 2019, so it is not surprising that we see different neutralising antibody levels against these new variants.

"No vaccine is 100% effective, so it will take more of us to be fully vaccinated to bring the spread of this newest variant under control."

For the Sars-CoV-2 Legacy study, led by the Crick and partners at UCL and University College London Hospitals NHS Foundation Trust (UCLH), healthcare workers and staff from the institutions have been donating regular blood and swab samples so researchers can track changing risk of infection and response to vaccination.

The people analysed in this part of the study were younger than average Oxford-AstraZeneca vaccine recipients (median age 34), so more research is needed to understand the antibody response in older people.