A single dose of Pfizer/BioNTech vaccines against COVID may boost immunity against the coronavirus variants, only in people who were previously infected with the deadly virus, a study has found, highlighting the importance of a second dose.

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In people who have not previously been infected and have so far only received one dose of vaccine, the immune response to variants of concern may be insufficient, said the team of researchers at Imperial College London, Queen Mary University of London and University College London. The study, published in the journal Science, looked at immune responses in UK healthcare workers at Barts and Royal Free hospitals following their first dose of the Pfizer/BioNTech vaccine. People who had previously had mild or asymptomatic infection had significantly enhanced protection against the UK and South Africa variants, after a single dose of the mRNA vaccine.

In those without prior COVID-19, the immune response was less strong after a first dose, potentially leaving them at risk from variants. "Our findings show that people who have had their first dose of vaccine, and who have not previously been infected with SARS-CoV-2, are not fully protected against the circulating variants of concern", said Rosemary Boyton, Professor of Immunology and Respiratory Medicine at Imperial College. "This study highlights the importance of getting second doses of the vaccine rolled out to protect the population", Boyton added.

Blood samples were analysed for the presence and levels of immunity against the original strain of SARS-CoV-2, as well as the UK (B117) and South Africa (B1351) variants of concern. Along with antibodies, the team also focused on two types of white blood cells: B-cells, which 'remember' the virus; and T cells, which help B cell memory and recognise and destroy cells infected with the coronavirus. They found that after the first dose of the vaccine, prior infection was associated with a boosted T cell, B cell and neutralising antibody response, which could provide effective protection against SARS-CoV-2, as well as the UK and South Africa variants. However, in people without previous SARS-CoV-2 infection, a single vaccine dose resulted in lower levels of neutralising antibodies against SARS-CoV-2 and the variants, potentially leaving them vulnerable to infection and highlighting the importance of the second vaccine dose.