Risks to herd immunity
Herd immunity requires that enough people be simultaneously immune to SARS-CoV-2 to prevent widespread ongoing transmission. While data indicate that the most likely scenario is to reach this state on the timelines described above, five risks could delay progress.
First, vaccine adoption may prove lower than expected. That could happen if a real or perceived safety issue increases hesitancy or if younger populations see little reason to be vaccinated once older cohorts are protected and a transition toward normalcy is well underway. Second, herd immunity relies on the efficacy of vaccines at reducing transmission (rather than the usually reported efficacy at preventing disease in the vaccinated person). While initial data suggest that COVID-19 vaccines do block significant transmission,25 the efficacy rate may not prove high enough to drive herd immunity. Third, the duration of vaccine-mediated immunity may prove shorter than anticipated, making it hard to reach the necessary threshold for simultaneous immunity. Fourth, supply-chain disruptions and delays are real, and could produce supply shocks and interfere with timelines. Fifth, and most concerning, variants that reduce the efficacy of vaccines or the benefits of natural immunity may spread widely. Some initial data offer concerning evidence that B.1.351 and P.1 may be examples of such variants, although recent Novavax data (with a small sample size) offer some reassurance that its vaccine is effective against severe disease caused by B1.351.26 Similarly, limited data from the Pfizer-BioNTech and AstraZeneca vaccines show evidence of some protection against P.1.27
McKinsey’s evolving perspectives on when will the COVID-19 pandemic end in the United States updated to examine how timing will vary around the world.
www.mckinsey.com