The charts above, from the Warwick paper, show the implications for unwinding social distancing in Britain.
If Boris Johnson were to end the national lockdown in mid-February, when only the top four priority groups have been vaccinated, and revert to the restrictions in place in early September, deaths would simply surge again even if the vaccines proved 85 per cent effective at blocking transmission (see first chart).
The last two charts assume a more gradual easing of restrictions. This keeps daily deaths manageable with a vaccine that is 85 per cent infection blocking but leads to a large spike in deaths in all other scenarios. This, say the authors, is because less efficacious vaccines have only a moderate impact on the ‘R’ rate and so community infections continue to climb.
It is only in the scenario where lockdown is maintained until the end of May (second chart), and then only relaxed to September’’s rules, that deaths are kept low for all vaccine transmission efficacy levels other than zero.
The row that exploded between the EU and UK over vaccine supply last week suggests that governments know which vaccines prevent transmission (why else would sensible politicians risk alliances given the stakes involved?) but the truth is no one yet has the faintest clue. The impact of vaccines on transmission remains one of the most important “known unknowns” of the crisis.
There are some small pointers. It is known, for instance, from early animal studies that the Oxford AstraZeneca vaccine did not prevent monkeys from becoming infected with the virus after vaccination, although it did stop severe disease.
The Telegraph also knows of a nurse working in a special needs hospital in Norfolk who received the vaccine Pfizer vaccine before Christmas but this week tested positive for the virus. She is asymptomatic but, because it is not known if her infection is transmissible, she has had to stop work and self-isolate.
Another clue is provided by the trials of the Chinese Sinovac vaccine in Brazil. While most phase three vaccine trials only looked for people who developed symptoms of Covid-19, this one tracked positive asymptomatic participants. When this group was included, efficacy at preventing infection fell from 78 to 50 per cent.
Dr Sam Moore, a postdoctoral research associate at Warwick and one of the modelling study’s authors, says most experts expect vaccines to provide some protection against transmission but at “moderate levels”, which would be likely to differ between vaccines.
“It doesn’t seem reasonable you would get vaccines which are so effective at preventing disease without having some impact on infectivity”, he said. “On the other hand, transmission-blocking is not going to be higher than a vaccine’s ability to protect against severe disease. We think 60 per cent transmission-blocking is possible, but we really don’t know.”