Matthew Conlen and Charlie Smart,
New York Times Service
February 23, 2021
With the vaccine rollout underway and coronavirus cases declining after a dark winter surge, it may seem as if the end of the pandemic is in sight. In reality, how soon could we get there?
One answer lies in herd immunity, the point when enough people are immune to the virus that it can no longer spread through the population. Getting there, however, depends not just on how quickly we can vaccinate but on other factors, too, like how many people have already been infected and how easily the virus spreads.
The current path to herd immunity in the United States, based on a model developed by PHICOR, a public health research group, looks at the number of people who have been fully vaccinated and combines that with an estimate of the number of people who have been infected and have recovered to measure total immunity.
The exact threshold for herd immunity for the coronavirus is unknown, but recent estimates range from 70% to 90%.
At first, the model looks like pretty good news — under these assumptions, we could reach herd immunity as early as July. But a lot could happen between now and then. The speed and uptake of vaccination, and how long immunity lasts are big factors. The rise of new virus variants and how we respond to them will also affect the path to herd immunity.
In most scenarios, millions more people will become infected and tens or hundreds of thousands more will die before herd immunity is reached.
What if We Speed Up Vaccinations?
More than 15 million people have been fully vaccinated, and the U.S. is currently administering about 1.7 million shots per day. Some experts say we could nearly double that pace by April as new vaccines are approved. (Because the current vaccines require two doses spaced weeks apart, the number of people fully vaccinated each day is smaller.)
The more people we vaccinate, the faster we could reach the threshold for herd immunity.
It’s important to note that total immunity relies on an estimate of who has already been infected, including people who have immunity from undiagnosed cases. But in a situation in which a new but less effective vaccine arrives, we might not reach the threshold through vaccination alone.
And the model comes with some other caveats. Much is still unknown about how long immunity from vaccines will last or how well the vaccines will protect against new variants of the virus. The estimates also assume that the vaccine prevents infection rather than just reducing the severity of coronavirus symptoms.
“There are still key pieces of missing information that could substantially affect what may happen to the pandemic over the ensuing months,” said Dr. Bruce Y. Lee, a professor at the City University of New York School of Public Health & Health Policy who led the research effort for PHICOR. “Should many people lose immunity over the next several months after having recovered from infections, that would make many more people susceptible to the virus again.”
Some experts argue that reducing deaths and severe illness is a better and more achievable goal than full herd immunity, and ramping up vaccinations is still the best way to do that.
What if We Relax Social Distancing?
Measures like wearing masks and social distancing have proved to be effective in slowing the spread of the virus. As more people get vaccinated, it’s tempting to ease up on those precautions.
While this means we would reach herd immunity sooner because more people would get sick and develop antibodies, that speed would come at a grim cost.
Precautions remain especially important as new variants of the virus emerge. If social distancing measures aren’t followed, a stronger virus could rapidly infect and kill hundreds of thousands of people before they can be vaccinated.
What if a More Contagious Variant Spreads?
A more contagious variant of the virus, like the one first identified in Britain that is now spreading throughout the United States, could further complicate the path to herd immunity.
If the virus becomes more contagious, the threshold for herd immunity will go up. It may be hard for vaccines to keep pace, and precautions will be even more necessary to stem the spread.
The spread of new virus variants makes it impossible to put a firm date on when we’ll reach herd immunity or when the pandemic will end. There’s a chance a mutation could lead to a version of the virus that doesn’t respond to existing immunity, leading us to start the journey to herd immunity all over again.
Lee said that coronaviruses have relatively high mutation rates and that it is likely that new variants will continue to emerge. “The question will be how different might these variants be,” he said. If a variant of the virus stops responding to the vaccine, “it will then be a matter of determining whether and when new vaccines will need to be produced.”
The model used in this article was adapted from one originally published in the American Journal of Preventive Medicine and has been updated to reflect the most recent estimates of the prevalence, transmissibility and severity of the coronavirus as of Feb. 16.
To adapt the model for the web, The New York Times worked with researchers from Public Health Informatics, Computational and Operations Research, or PHICOR: Bruce Y. Lee, Sarah M. Bartsch, Kelly J. O’Shea, Patrick T. Wedlock and Marie C. Ferguson. PHICOR is headquartered at the City University of New York School of Public Health & Health Policy.
Historical case data, the best available estimates of the transmissibility of the virus and the impact of social distancing measures are used to estimate the future trajectory of new cases and deaths as the vaccine is administered. Vaccine immunity represents the share of people who are fully vaccinated and have immunity, taking into account the efficacy of the available vaccines.
The model assumes that immunity lasts indefinitely and that vaccination prevents infection rather than just reducing the severity of coronavirus symptoms. It does not account for the immunity that may be gained from only a single dose of vaccine.
It is assumed that the vaccine prevents infection against current and future coronavirus variants in 90% of those who are fully vaccinated and that 80% of the population is ultimately fully vaccinated.
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