What is the effectiveness of a single dose of the Pfizer-BioNTech vaccine?

The mRNA Pfizer-BioNTech vaccine was the first vaccine against SARS- CoV- 2 to be made available. Two doses of the vaccine confer 95% protection against the original Wuhan strain of the virus. But this effectiveness is lessened if both vaccines are not given, and depends on the nature of variants. The UK took the decision to vaccinate a maximum number of people with a first dose, increasing the delay period between the first and second doses. At the same time, the emergence of variants such as the Alpha (UK) and Beta (South African) changed the situation. British researchers (at Imperial College London, Queen Mary University of London, and the NHS Trust in London) have evaluated immune responses after just one dose of Pfizer-BioNTech.

The researchers examined B lymphocyte (which produce antibodies), T lymphocyte and neutralising antibody responses in a cohort of English health professionals who had received a single dose of the vaccine. Some had already been infected by the original strain of SARS-CoV-2 while others had not (infection-naïve individuals).

70% of vaccinated naïve individuals developed a T lymphocyte response against the spike (S) surface protein of the virus, as opposed to 90% of infected and vaccinated individuals. Similarly, the B lymphocyte response was significantly greater in previously infected vaccinated individuals than in naïve individuals. Finally, those individuals vaccinated after natural infection produced more neutralising antibodies.

As regards the immune response to variants after a single dose, it appears that the majority of naïve individuals did not develop antibodies against the Alpha and Beta variants, unlike previously infected vaccinated individuals, who developed a strong antibody response against both strains. The T lymphocyte response against these variants was more diverse.

This study showed that a single dose of the Pfizer-BioNTech vaccine enables the immunity of people having already been infected naturally by SARS-CoV-2 to be reinforced, and significantly increases protection against variants. It is important now to monitor the duration of this immunity as well as any immune system escape potential in new emerging variants.