Pfizer conducted a randomised controlled trial across several countries.7 In patients who had received two doses of the vaccine, efficacy against COVID-19 was 95%. After only one dose, efficacy was 52%, indicating early protection, beginning 12 days after the first dose. About 40% of the Pfizer trial cohort were aged over 55 years. Vaccine efficacy in this age group was 93.7%.7
The Oxford-AstraZeneca vaccine phase III trial results are more complicated, derived from pooled data from four individual trials that tested different vaccine doses and schedules across three countries.8 Efficacy in this trial has been widely quoted as 70.4%, but this was a combination of efficacy in people who received two standard doses (62.1%) and efficacy in those who received a low dose followed by a standard dose (90%).8 As the latter dose change was a protocol error during the trial, this dosing regimen needs to be further validated before it is implemented.8,11
Further complicating results was variation in the interval between doses. An analysis of follow-up trial data found that vaccine efficacy with two standard doses was 55.1% in people with a dose interval less than six weeks but 81.3% in those with a dose interval of 12 weeks or more.11
A single dose of the Oxford-AstraZeneca vaccine had an efficacy of 76% against symptomatic COVID-19 in the first 90 days after vaccination.11 However, it is unclear at what point the first dose ‘becomes’ effective, although a recent analysis of population data from Scotland after the introduction of mass vaccination showed some efficacy against COVID-19 hospitalisation from day 7 after vaccination (70%) and peak efficacy at days 28 to 34 (94%).15 Similar population studies after widespread vaccination in England found that a single vaccine dose conferred a 37% lower risk of emergency hospitalisation.16 Further evidence is needed to clarify protection against asymptomatic infection.16
Media releases have reported that the Novavax vaccine had an efficacy of 89.3% in phase III trials in the UK.9 About one-third of the cohort was aged over 65 years, but we do not yet know how effective the vaccine was in this age group.
Although all three vaccines appear to protect against the development of severe COVID-19, commonly defined as COVID-19 respiratory infection requiring hospitalisation, there were few cases of severe COVID-19 in the phase III trials in either the vaccine or placebo groups.7,8 ‘Real world’ data from population studies across England and Scotland suggest that with a program of vaccination with the Pfizer and Oxford-AstraZeneca vaccines, a single dose of either vaccine is at least 80% effective at preventing hospitalisation with severe disease.15,16
Overall, the Pfizer and Oxford-AstraZeneca vaccines appear safe and well tolerated, with extremely low rates of serious adverse events. Mild adverse effects are common, with pain at the injection site and flu-like symptoms being those most reported (Table 2).7-9,13,14
Data have not yet been released on the adverse effects of the Novavax vaccine. However, press releases have stated it was well tolerated in clinical trials.