“Herd immunity occurs when a large portion of a community (the herd) becomes immune to a disease, making the spread of disease from person to person unlikely. As a result, the whole community becomes protected — not just those who are immune.”
– Mayo Clinic (December 15, 2020)
There are common features in how any infection spreads. When enough people are immune – through vaccination or natural immunity – a population achieves herd immunity. The disease stops spreading efficiently and starts to fade away.
“Vaccines train our immune systems to create proteins that fight disease, known as ‘antibodies’, just as would happen when we are exposed to a disease but – crucially – vaccines work without making us sick. Vaccinated people are protected from getting the disease in question and passing on the pathogen, breaking any chains of transmission.”
– “Coronavirus disease (COVID-19): Herd immunity, lockdowns and COVID-19.” World Health Organization. December 31, 2020
There are two paths to herd immunity for COVID-19 – vaccines and infection. How quickly a community gets to herd immunity depends on a number of factors.
What will it take to finally halt the spread of the coronavirus in the U.S.? To answer that question Thomas Wilburn and Richard Harris of NPR created a simulation of a mock disease they're calling SIMVID-19. They chose to simulate a fake disease since there are too many unknowns to simulate the course of COVID-19.
Here is what they found:
1. A more infectious variant takes over
It looks as if it will take higher vaccination rates to achieve herd immunity against the more infectious strain of SIMVID-19. This is a real concern with emerging variants of the coronavirus, including the strain first detected in the U.K., which is now spreading rapidly in the U.S.
There's always an element of chance in how diseases spread, as you can see each time you re-run these simulations.
2. A population is already heavily exposed
The pandemic has hit some communities harder than others, leaving in its wake some degree of natural immunity. This scenario assumes a lot of people are already immune to SIMVID-19, through a previous infection.
Higher levels of immunity — along with a high vaccination rate — would help a community reach herd immunity. For the coronavirus, we don't know what percentage of people who have been infected are immune, or how long that immunity lasts.
3. A population has low levels of initial immunity
SIMVID-19 can spread more easily in a population where few people are immune at the start of an outbreak. It is harder to achieve herd immunity in this scenario, and increased levels of vaccination are key. When it comes to coronavirus, some communities have lower level of preexisting immunity and scientists think that the immunity people do have may fade over time.
PLEASE NOTE: The NPR article “How Herd Immunity Works ...” offers a very interesting simulation of the mock disease they're calling SIMVID-19. When you click "Run Simulation,” you can witness how a disease can spread through a population and how increased levels of vaccination can stop it in its track. I urge you to read the article and run the simulation for yourself. Click here: https://www.npr.org/sections/health-shots/2021/02/18/967462483/how-herd-immunity-works-and-what-stands-in-its-way
What Does This Mean?
Takeaways for COVID-19? In SIMVID-19 scenarios, 75% vaccination rates were able to kick the imaginary disease in all but one scenario, the more infectious variant. For the U.S. to tame its coronavirus outbreak through herd immunity, scientists expect that somewhere between 70% and 85% of the population must be immune.
However, no vaccine is 100% effective, so some vaccinated people will still get ill and could still pass on the disease. Also, we don't know how long immunity following infection will last. Finally, there is a concern that some variants now spreading around the world will render vaccines less effective.
(Thomas Wilburn and Richard Harris. “How Herd Immunity Works — And What Stands In Its Way.” NPR. February 18, 2021.)
How long will it take to reach herd immunity in the U.S.?
It depends on a lot of things. Perhaps most importantly, it depends on how willing the public is to get vaccinated and how quickly it can be done.
Some basic math and assumptions paint the picture:
We need to average 2.4 million doses a day starting now to reach the point where 70% of the population is vaccinated by July 4th (assuming two doses needed per person). There are many estimates out there of what’s needed for herd immunity, and that’s probably the bare minimum. It’s also harder than it sounds, because kids aren’t being vaccinated right now, so we need to reach the vast majority of adults, which means overcoming hesitancy where it exists.
It’s 1.9 million doses to reach it by Labor Day.
And 1.2 million doses per day if we achieve the goal by January 1, 2022.
(Drew Altman. “How Quickly We Need To Ramp Up Vaccinations To Get To Herd Immunity. KFF COVID-19 Vaccine Monitor. January 26, 2021.)
The limiting factor may be the supply of vaccine. That too seems somewhat hopeful with J+J/Janssen and then others from AstraZeneca and Novavax expected to come on line. And, the J&J vaccine is expected to be a single dose rather than two, so it would mean fewer overall doses are needed. Still, the biggest mystery remains what the supply of vaccine is expected to be and when new approved vaccines will be ready, even if everything breaks favorably.
How do we know we've reached herd immunity? Proof that we’re nearing herd immunity would be a “disruption in the chain of transmission,” said Ashley St. John, who studies immune systems at Duke-NUS Medical School at Singapore.
To determine whether to relax restrictions, health officials will be watching infection and hospitalization trends as vaccinations roll out. And those decisions are likely to begin long before the ideal herd immunity threshold is reached, though they will be gradual and vary by region.
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