With vaccine implementation underway and coronavirus cases declining after a black winter, it may appear that the end of the pandemic is near. In fact, when could we get there?
One answer is collective immunity, the point at which enough people are immune to the virus and it can no longer spread to the population. Getting there, however, depends not only on how quickly we can vaccinate, but also on other factors, such as how many people have been infected and how easily the virus spreads.
An estimate for the path to herd immunity
This chart shows the current path to herd immunity in the United States, based on a model developed by PHICOR, a public health research group. He looks at the number of people who were fully vaccinated and combine this with an estimate of the number of people who were infected and recovered to measure total immunity.
When the orange line crosses to the blue area, it means that we have entered the herd immunity range. The exact limit of collective immunity for coronavirus is unknown, but recent estimates range from 70% to 90%.
At first, this looks like very good news – under these premises, we could achieve collective immunity as early as July. But a lot can happen between now and then. The speed and application of vaccination and the duration of immunity are important factors. The emergence of new virus variants and how we respond to them will also affect the path to collective immunity.
In most scenarios, millions of people will be infected and tens or hundreds of thousands will die before collective immunity is achieved.
What if we speed up vaccinations?
More than 15 million people have been fully vaccinated and the United States currently administers around 1.7 million vaccines per day. Some experts say we can almost double that pace until April, when new vaccines are approved. (As current vaccines require two doses at intervals of weeks, the number of people who are fully vaccinated each day is less.)
The more people we vaccinate, the faster we can reach the collective immunity limit.
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If the pace increases to 3 million shots a day, we can reach the herd’s immunity limit in May. In that time, 90,000 people can die from the virus.
It is important to note that the orange line for total immunity depends on an estimate of who has already been infected, including people who have immunity from undiagnosed cases. We can be more sure that we will achieve collective immunity when the pink vaccination line reaches that range. However, in a scenario where a new but less effective vaccine arrives, we may not reach the limit with vaccination alone.
And the model comes with some other caveats. Much is still unknown about how long the vaccine’s immunity will last, or how well the vaccines will protect against new variants of the virus. Estimates also assume that the vaccine prevents infection rather than just reducing the severity of coronavirus symptoms.
“Important information is still lacking that could substantially affect what could happen to the pandemic in the months ahead,” Dr. Bruce Y. Lee, professor of health policy at City University of New York who leads the research effort for PHICOR, said . “If many people lose immunity in the next few months after they have recovered from infections, many more people will be susceptible to the virus again.”
Some experts argue that reducing deaths and serious illnesses is a better and more attainable goal than full herd immunity, and increasing vaccinations is still the best way to do this.
What if we relax social distance?
Measures such as wearing masks and social distance have proven to be effective in delaying the spread of the virus. As more people are vaccinated, it is tempting to lessen these precautions.
While this means that we would achieve collective immunity sooner because more people would get sick and develop antibodies, that speed would come at a terrible cost.
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If we end the restrictions in April, we can reach the herd immunity limit in June. But at that time, 170,000 more people can die from the virus.
Precautions remain especially important as new variants of the virus emerge. If social distance measures are not followed, a stronger virus can quickly 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 across the United States, could further complicate the road to collective immunity.
If the virus becomes more contagious, the limit for collective immunity will increase. It can be difficult for vaccines to keep pace, and precautions will be even more necessary to stem the spread.
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A more contagious variant will spread more quickly and increase the herd’s immunity limit. We can still hope to achieve herd immunity in July, but new deaths may double, to 200,000 people.
The spread of new virus variants makes it impossible to predict when we will achieve collective immunity or when the pandemic will end. There is a chance that a mutation could lead to a version of the virus that does not respond to existing immunity, prompting us to start the journey to collective immunity again.
Dr. Lee said that coronaviruses have relatively high mutation rates and that it is likely that new variants of the virus will continue to emerge. “The question will be how different these variants can be,” he said. If a variant of the virus stops responding to the vaccine, “it will be a matter of determining whether and when new vaccines need to be produced”.
Methodology
The model used in this article was adapted from one originally published in the American Journal of Preventive Medicine and was updated to reflect the most recent estimates of the prevalence, transmissibility and severity of the coronavirus on February 16.
To adapt the model for the web, The New York Times worked with researchers in Public Health Informatics, Computational and Operational Research or PHICOR: Bruce Y. Lee, Sarah M. Bartsch, Kelly J. O’Shea, Patrick T. Wedlock and Marie C. Ferguson.
Historical case data, the best available estimates on the transmissibility of the virus and the impact of social distance measures are used to estimate the future trajectory of new cases and deaths as the vaccine is administered. Vaccine immunity represents the proportion of people who are fully vaccinated and have immunity, taking into account the effectiveness of 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 take into account the immunity that can be achieved with just a single dose of the vaccine.
The vaccine is presumed to prevent infection against current and future variants of the coronavirus in 90% of people who are fully vaccinated and 80% of the population is fully vaccinated.