Explainer: How effective will the coronavirus vaccines be?

New Delhi, Nov 20: Multiple COVID-19 vaccines are currently in phase 3 trials with efficacy assessed as prevention of coronavirus disease.

A popular assumption is that these vaccines will provide population immunity that can reduce transmission of the infection.

WHO recommends that successful vaccines should show disease risk reduction of at least 50 per cent, with 95 per cent CI that true vaccine efficacy exceeds 30 per cent.

However, the impact of these COVID-19 vaccines on infection and thus transmission is not being assessed. Even if vaccines were able to confer protection from disease, they might not reduce transmission similarly.

There have been reports of virologically confirmed that re-infection of previously infected individuals, but the extent of such re-infection is unclear. Whether re-infection is associated with secondary spread is unknown.

Encouraging results on Covid-19 vaccine trials by frontrunners in November have raised hopes that a first generation of coronavirus antidotes is likely to receive approval as soon as December end or early 2021.

Ongoing late-stage human trials of vaccine candidates developed by Pfizer-BioNTech and Moderna Inc have pointed to efficacy rates as high as 95 per cent, sowing hopes across the world in the battle against the pandemic that has claimed 1.34 million lives and upended economies everywhere.

Russia has also claimed a 92 per cent effectiveness for its Sputnik V vaccine candidate, which is being tested on humans in mid- to late-stage trials in India.

Notably, influenza vaccines are less effective in older populations than in younger populations, partly due to immune senescence, which might similarly affect COVID-19 vaccines.

What such vaccines are likely to achieve might not be herd immunity. If so, strategies for how to use such vaccines would have to be based on other considerations.

These observations suggest that we cannot assume COVID-19 vaccines, even if shown to be effective in reducing severity of disease.