31 Jan, 2021 12:00 AM4 minutes to read

Covid-19 vaccines will not only be judged on their ability to prevent disease. Photo / AP

There is strong evidence from clinical trials that the Covid vaccines being rolled out across the globe will prevent severe illness.

The trials found vaccines were up to 90 per cent efficacious in this respect and already there is early data that give cause for hope this will be replicated in the “real world”.

An Israeli study published last Wednesday of more than 500,000 people found a 51 per cent reduction in infections 13 days after people received their first dose of the Pfizer vaccine.

The results were broadly consistent across all age, sex, social and comorbidity groups, except for those with diabetes where protection was not as good. “We demonstrated an effectiveness of 51 per cent of [Pfizer] BNT162b2 vaccine against SARS-CoV-2 infection 13-24 days after immunisation with the first dose. Immunisation with the second dose should be continued to attain the anticipated [90 per cent] protection,” concluded the study’s authors.

But vaccines will not only be judged on their ability to prevent disease. Two other forms of efficacy are important: their ability to protect against asymptomatic infections and, crucially, their ability to stop transmission – the ability of the virus to spread from one person to another.

Unfortunately, the often counterintuitive rules of contagion mean the effectiveness of each vaccine in blocking transmission will determine a very large part of their worth when it comes to easing lockdowns and other social distancing restrictions.

At a national level, it is probably not an exaggeration to say it may make the difference between open and closed, boom or bust. For individuals, it could make the difference between a vaccine passport that provides the freedom to travel and mix and one that is all but useless.

Modelling commissioned by SPI-M, a subgroup of Sage, suggests the UK will suffer a third spike in deaths unless vaccination cuts transmission of the virus by a massive 85 per cent in those vaccinated.

“Our predictions highlight the population-level risks of early relaxation leading to a pronounced wave of infection, hospital admissions and deaths,” warns the paper produced by modellers at the University of Warwick.

“Only vaccines that offer high infection-blocking efficacy with high uptake in the general population allow relaxation of Non-Pharmaceutical Interventions (NPIs) without a huge surge in deaths”.

The finding, which has been passed up the chain to Downing Street, is not a one-off. Other modelling groups – including Imperial College London and the London School of Hygiene and Tropical Medicine (LSHTM) – have reached similar conclusions.

“It will really depend on what happens with vaccine roll-out speed, uptake, and efficacy against infection (i.e. ‘infection blocking’)”, said Adam Kucharski, an associate professor in mathematics and epidemiology at LSHTM.

“In a scenario where vaccines don’t prevent many infections and not everyone gets vaccinated, then transmission would continue, putting everyone who’s unvaccinated – or not totally protected from disease following vaccination – at ongoing risk.”

Modelling suggests the UK will suffer a third spike in deaths unless vaccination cuts transmission of the virus by a massive 85 per cent in those vaccinated. Photo / 123RFThis all shows the implications for unwinding social distancing in Britain. If UK Prime Minister Boris Johnson were to end England’s national lockdown in mid-February when only the top four priority groups have been vaccinated and revert to the restrictions that were in place in early September, deaths would simply surge again even if the vaccines proved 85 per cent effective at blocking transmission.

The impact of vaccines on transmission remains one of the most important “known unknowns” of the crisis.

There are some basic 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 relieve their symptoms.

The Sunday Telegraph also knows of a nurse working in a special needs hospital in Norfolk who received the 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.

A nurse in Norfolk tested positive for Covid-19 after receiving the Pfizer vaccine. Photo / APAnother 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 were included, efficacy at preventing infection (not necessarily the same as transmission) 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”.