If it behaves like influenza and other coronaviruses and starts accumulating minor mutations (a phenomenon called antigenic drift), then the virus could require yearly immunizations to cover new strains, similar to the flu. One is that the coronavirus does not mutate. This model, like every model, makes some assumptions. High confidence in a vaccine (64% get vaccinated by May 2021) Medium confidence in a vaccine (61% get vaccinated by May 2021) Low confidence in a vaccine (39% get vaccinated by May 2021) If 17.7% of Americans are already immune by May, and another 39% would readily get a Covid-19 vaccine, that leaves about 43% of the population who are vulnerable to infection but skeptical of vaccination.Ĭonvincing roughly half of these skeptical Americans to take the shot, boosting P v to 60.7%, shortens the time to herd immunity to two months, meaning July 2021.Ĭonvincing just a few more, hitting a critical mass of 63.7%, would let us achieve herd immunity as soon as the second wave of vaccinations has been completed.Įngage ‘willing skeptics’ to help increase Covid-19 vaccination rates Scenario With just 39% of Americans getting a government-approved vaccine, the time to herd immunity is 19 months, meaning December 2022. We’re almost there: Calling the percentage of Americans who get the vaccine P v and considering that 90% of people who get vaccinated will develop immunity, here’s an equation for an estimate of time to herd immunity starting from May 2021, using 39% as P v - percentage of Americans who said they would get a vaccine in an NBC News poll: At that point, in May 2021, the base prevalence of infection will be 17.7% (1.2% per month from October 2020, when the base prevalence was 9.3%).
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Since both vaccine candidates are a two-shot series separated by three to four weeks, it may take at least an additional month to gain full immunity. Anthony Fauci has predicted that Covid-19 vaccines, one of which is being rolled out this week to frontline health care workers, should become widely available to the public by April 2021.įor the sake of simplicity, I’ll ignore the phased rollout and imagine that all Americans have the choice to get vaccinated this April. Through a miraculous feat of science, we have a shortcut to herd immunity: two vaccines with over 90% efficacy, and possibly more on the way. This white-knuckle approach would consist of several years of misery, morbidity, and mortality, not to mention continued economic hardship. Even if I had assumed an R 0 of 3, it would still take 48 months to reach herd immunity. If the virus is left to spread at its current rate with no vaccine, it would take 55 months from October 2020 to achieve herd immunity. Using the herd immunity threshold, the base prevalence, and the monthly infection rate, it’s possible to calculate the number of months ( m) to achieve herd immunity: from February 27 to September 30, or an infection rate of 1.3% per month. This back-of-the-envelope calculation is in line with estimates from the medical literature, with one study estimating 52.9 million infections in the U.S. If the base prevalence at the end of September - eight months from the onset of the epidemic in the United States on Januwas 9.3%, the coronavirus has an infection rate of approximately 1.2% of the population per month. To be sure, antibody testing may suffer from low positive predictive value when the prevalence of infection is low, but this is the best estimate we have so far. In late September, a Stanford study estimated that 9.3% of Americans have antibodies against SARS-CoV-2. We cannot rely on magical thinking: Herd immunity is not a plan If R 0 is 4, then 75% of the population needs to acquire immunity to the virus in order to halt transmission. Using just the basic reproduction number, it’s possible to calculate the percentage of people needed to achieve herd immunity: This is the percentage of people who become infected and acquire immunity to the virus every month. This is the percentage of people immune to the virus at a given moment in time, either from acquired infection or vaccination. For the sake of this thought experiment, I assumed an R 0 of 4.īase prevalence. Currently, most estimates of R 0 are between 2.5 and 4. An R 0 of 2 means one individual infected with SARS-CoV-2 is likely to infect two other people. This number represents how infectious a pathogen is.
The basic reproduction number R 0 (pronounced R-naught). Jones, a family medicine physician at my hospital who made some predictions on the time required to achieve herd immunity based on vaccine adoption, I set out to answer my question with a model that uses the following variables and definitions: Exclusive analysis of biotech, pharma, and the life sciences Learn More
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