Two doses of the Oxford/AstraZeneca coronavirus vaccine induces lower levels of antibodies against the variant first detected in India than against other strains, new research suggests.

The laboratory findings from the Francis Crick Institute and the National Institute for Health Research (NIHR) UCLH Biomedical Research Centre compared the results to their findings in relation to the Pfizer/BioNTech jab.

The data suggests that both vaccines induce lower levels of antibodies targeting the Delta (B.1.617.2) variant.

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

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

The research also suggests antibody levels induced by the Oxford/AstraZeneca vaccine vary depending on likely prior infection.

Researchers observed that people who had previously reported Covid-19 symptoms, had higher antibody levels after their first vaccine dose than those who did not report symptoms and did not if they had previously had the disease.

They compared concentrations of neutralising antibodies across all variants.

Data from previous studies suggests the higher antibody titres – the greatest dilution level that still blocks 50% of virus infection in the lab – is a good predictor of vaccine efficacy and greater protection against the virus.

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.

In people who had only received one Oxford jab, levels 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.

Researchers say a significant proportion of people without prior symptoms had antibody levels below the limit of detection against new variants of concern – 65% against B.1.1.7, 88% against B.1.351, and 85% against B.1.617.2.

This variation was not seen in recipients of the Pfizer vaccine, according to the research letter published in The Lancet.

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.”

As part of 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 that 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.