A nurse at the Royal Free Hospital preps a Covid-19 vaccine - Credit: PA/Yui Mok Staff at the Royal Free played a key role in a Covid-19 vaccine study which found that a single dose can significantly boost how protected someone is against new variants of the virus. If someone has already had mild or asymptomatic Covid, then research on NHS workers - including 200 at the Royal Free - showed a single dose of the Pfizer/BioNTech vaccine conveys "significantly enhanced" protection against the coronavirus - including the variants first identified in Kent and South Africa. The immune systems of those who had not previously had Covid-19 responded less strongly to the first dose of the vaccine - highlighting the importance of second doses in fully protecting people.