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US herd immunity: Still on track for 2021, but new variants threaten the timeline

While the probability of achieving herd immunity by fall is still the most likely scenario, the rise of more contagious mutations and slower initial vaccine rollout increase the likelihood that the UK and the US will acquire herd immunity by early 2022.

Probability of herd immunity to COVID-19 for the United Kingdom and United States, by date; factors affecting the date (chart)

More-transmissible variants and slower initial vaccine rollout increase downside risk to the Q3–Q4 2021 timeline to achieve herd immunity.

Chart: Probability of herd immunity1 to COVID-19 pandemic for the United Kingdom and United States2 by quarter (illustrative)

Two skewed bell curves estimate when herd immunity will occur.

  • 11/23/20 estimate. From Q4 2020 to Q1 2021 the probability of herd immunity is negligible. The curve rises sharply from Q1 2021 to a peak between Q3 and Q4 2021. The long tail of the curve shows falling probabilities to Q3 2023 and beyond.
  • 1/15/21 estimate. The start, peak, and tail of this curve are almost the same as the previous one. A flatter curve indicates herd immunity will probably occur one month later.

Early herd immunity if:

  • Vaccine rollout and adoption are faster than expected
  • Natural immunity is significantly higher than realized
  • More-transmissible variants lead to higher rates of natural immunity

Peak probability of herd immunity driven by:

  • US Biologics License Applications (BLA) with full approval by March/April 2021 or earlier
  • Approximately 3–9 months for manufacturing, distribution, and sufficient adoption to reach herd immunity

Later herd immunity if one or more of the following occur:

  • Safety issues delay BLA
  • Manufacturing/supply-chain issues slow rollout
  • More-infectious variants raise the threshold for achieving herd immunity
  • Adoption is slower than anticipated
  • Duration of immunity is short
  • Vaccine prevents disease progression but does not meaningfully reduce transmission

Notes

1Herd immunity is achieved when a sufficient portion of a population is simultaneously immune to prevent sustained transmission. At this point, significant, ongoing public-health measures are not needed to prevent future spikes in disease and mortality (this might be achieved while there are still a number of people in particular communities who still have the disease, as is the case with measles).

2Timeline to functional end is likely to vary somewhat based on geography.

McKinsey & Company

To read the article, see “When will the COVID-19 pandemic end?,” January 20, 2021.