When Will We Reach Herd Immunity from COVID-19?

Archived: See https://www.chmura.com/blog/herd-immunity-may-2021 for an updated version of this forecast.

Model updated February 9, 2021.

As the number of new COVID-19 infections fell and vaccination rates ramped up over the past two weeks, cautious optimism for a return to normalcy has begun to spread among health experts. While there is abundant caution about assigning a population percentage to achieve herd immunity, many experts agree about 80% of the population will need to be immune to the virus to reach herd immunity, either through vaccination or by previously contracting the virus. So when will we get there?

As of February 8, 2021, over 32 million Americans have received their first dose of the Pfizer or Moderna vaccine, including over 9 million who have also received their second dose.[1] On average for the past week, over 1.4 million doses have been administered per day. Using vaccination rates from the Centers for Disease Control and Prevention (CDC), estimated infection counts from the University of Washington, vaccine allotment announcements, and information regarding vaccination rate ramp-up from President Biden, Dr. Fauci, and other medical experts, Chmura built a herd immunity timeline model with three scenarios.

In the optimistic scenario, the vaccination rate in the United States slightly exceeds President Biden’s 100-day goal of 100 million Americans receiving their first dose. In this scenario, vaccination rates scale up to nearly 3 million total doses per day in May 2021 as problems arising from vaccine distribution and administration are quickly resolved. After somewhat tapering off as the non-vaccinated population becomes more difficult to reach, over 80% of the U.S. population is fully vaccinated in September 2021. However, after accounting for individuals immune to COVID-19 via infection, herd immunity is reached one month earlier in August 2021.

In the most likely scenario, 87 million Americans receive their first dose of the COVID-19 vaccine by the end of President Biden’s first 100 days. Supply chain disruptions and difficult-to-reach populations constrain vaccine administration to a peak of only 2 million total doses administered per day. In this scenario, the nation fully vaccinates over 80% of its population by the end of November 2021.

In the pessimistic scenario, only 67 million Americans receive their first dose of the COVID-19 vaccine by the end of President Biden’s first 100 days. Significant supply chain disruptions and slower vaccine acceptance caps the average number of vaccine doses administered per day at 1.5 million. In this scenario, the nation does not fully vaccinate over 80% of Americans until April 2022.

The following chart shows the national ramp-up toward herd immunity from COVID-19.

Chmura will update this model at least weekly and will soon release state-level models.

Note that this model includes estimated infections of COVID-19. Individuals who contract COVID-19 are estimated to be immune from a second infection for the five months following their first infection.

Additional Assumptions of the Model

Status quo:

• 600 million total vaccine doses purchased from Moderna & Pfizer alone, including January 27, 2021 announcement of 200 million additional doses purchased.
• Over 32 million Americans have received their first dose, including over 9 million who have received their second dose, per the CDC as of February 8, 2021.
• The United States is currently administering about 1.4 million total doses per day, with President Biden expecting that to soon rise to 1.5 million per day (federal-to-state distribution plan is increasing from 8.6 million doses per week to 10+ million per week).

Broad Assumptions:

• States’ current vaccine administration figures are predictors of their ability to continue administering the vaccine in the future at a rate that eventually returns to the national average by the end of 2022.
• Future introduction of additional vaccines (e.g., Johnson & Johnson, AstraZeneca, Novavax) do not significantly alter the United States’ vaccine roll-out.
• 80% is the threshold for herd immunity, either via vaccine or antibodies via infection (assumed to remain in infected individuals for six months).
• 80%+ of Americans are (or become) willing to receive a vaccine (this number has been trending upwards over recent months; at 71% in December 2020, per Kaiser Family Foundation).
• Pfizer and Moderna vaccines maintain high levels of efficacy against new strains of the virus, including the variants from the United Kingdom, South Africa, Brazil, and California.

Optimistic

• Assumptions:
  • No significant supply chain disruptions.
  • 88%+ of Americans are willing to get vaccinated in the next 12 months.
  • 99% of Americans who receive the first dose return for their second dose.
• Slightly exceeds President Biden’s goal of 100 million vaccinated Americans in first 100 days (111 million with first dose, including 80 million with both doses).
• Consistent with the purchase of 200 million more doses of Pfizer/Moderna arriving on schedule, with full delivery expected in summer 2021, according to the White House. This purchase is in addition to the existing 400 million doses ordered from Pfizer and Moderna.
• Peaks near 3 million doses administered per day, on par with experts’ rosiest forecasts.
• 80%+ receive both doses by end of September 2021.

Most Likely

• Assumptions:
  • Moderate supply chain disruptions that slow the ramp-up for vaccine administration and cap doses per day near 2 million.
  • 85%+ of Americans are willing to get vaccinated in the next 12 months.
  • 98% of Americans who receive the first dose return for their second dose.
• Slightly falls short of President Biden’s goal of 100 million vaccinated Americans in first 100 days (87 million with first dose, including 72 million with both doses).
• 80%+ receive both doses by end of November 2021.
  
  

Pessimistic

• Assumptions:
  • Significant supply chain disruptions that cap doses per day under 1.5 million.
  • 82%+ Americans are willing to get vaccinated in the next 16 months.
  • 97% of Americans who receive the first dose return for their second dose.
• Falls well short of 100-day plan (67 million with first dose, including 60 million with both doses).
• 80%+ receive both doses by end of April 2022.

[1] Both the Pfizer and Moderna vaccines currently require two doses to reach maximum efficacy.