Health officials around the world are racing to vaccinate enough people to stop the spread of COVID-19, but what qualifies as “sufficient” is still an open question.
The goal is to achieve “herd immunity,” that is, when enough people have immunity, either to vaccination or previous infection, to stop uncontrolled spread.
Herd immunity doesn’t make anyone immune, and outbreaks can still flare up. It means that a virus no longer easily leaps from person to person, protecting those who are still vulnerable to catching it.
No one is sure what the herd immunity threshold is for the coronavirus, although many experts say it is 70% or higher. And the emergence of variants makes the picture even more complicated.
Here’s what is known about the virus and herd immunity.
HOW IS THE IMMUNITY THRESHOLD CALCULATED?
It’s a formula based on how contagious a virus is – or how many people on average contract the virus from one infected person.
But the calculation only provides a broad target for when a large spread can decrease. The figure may also vary by region.
“It’s not 64.9 is awful and 70.1 is fantastic,” said Dr. Walter Orenstein, an infectious disease expert at Emory University.
Orenstein notes that vaccination levels and other factors influencing spread can differ even within a city.
HOW DO WE KNOW WE HAVE ACHIEVED THE IMMUNITY OF THE HERD?
The evidence that we are approaching herd immunity would be a “disruption of the chain of transmission,” said Ashley St. John, who studies immune systems at Duke-NUS Medical School in Singapore.
But don’t wait for a big statement that we’ve reached that milestone.
To determine whether restrictions should be relaxed, health officials will monitor infection and hospitalization trends as vaccinations roll out. And those decisions will likely begin long before the ideal threshold for herd immunity is reached, although they will be gradual and vary by region.
In IndiaFor example, scientists believe that more people need to be protected in densely populated cities, where the virus spreads faster, than in the vast countryside.
India plans to search for antibodies nationally to find out what percentage of the nearly 1.4 billion people are already infected, said Dr. Jayaprakash Muliyil, who advises the government on virus surveillance.
The effectiveness of the vaccine also plays a role. Fewer people need to be vaccinated to achieve herd immunity if the shots are of higher effectiveness.
HOW DO CORONAVIRUS VARIANTS AFFECT THE IMMUNITY OF THE HERD?
It depends on the protection that a previous infection or vaccination gives you against the variant.
If vaccines were found to be significantly less effective against a variant, they would require an even greater proportion of the population to be vaccinated or existing vaccines updated to make them more effective, Orenstein said.
So far, the shots seem to offer at least some protection against the most troubling variants. But scientists are still studying the situation and are concerned about further mutations.
The variants have underscored the importance of vaccinating humans as soon as possible. Slowing the transmission is critical because viruses can mutate when they infect humans.
SHOULD THE IMMUNITY OF THE HERD BE WORLDWIDE?
Global herd immunity is ideal, but unlikely.
Rich countries have reserved most of the vaccines that will be manufactured this year. In the US, for example, officials have said that enough people can be vaccinated in the fall to return to normal.
But many poorer countries will likely have to wait longerThis is why the World Health Organization has warned that global immunity to the herd is unlikely to be achieved this year.
Differences in vaccination levels between countries are also why many experts believe the virus will never be completely eradicated.
CAN HERD WEAR IMMUNITY?
It is not known how long immunity lasts, either after vaccination or through infection, although experts believe it should last for at least several months.
Still, booster shots may be needed on the go. And while current COVID-19 vaccines are expected to work on the variants identified in the UK and elsewhere, the virus may mutate enough over time that recordings need to be updated.
Mutations in flu viruses, for example, are the reason why we get flu shots every year. But experts note that coronaviruses generally don’t mutate as readily.
WHAT IF COVID-19 VACCINES DO NOT PREVENT INFECTION?
The rollout of COVID-19 vaccines is now proving to be very effective in preventing people from getting sick. We do not know yet how well they can stop the infection completely, but they should help significantly reduce the spread of the virus.
That’s because the vaccines prevent the virus from multiplying in your body. So even if you become infected after vaccination, your body should be shedding less virus and for a shorter time, said Deborah Fuller, a vaccine expert at the University of Washington.
It is another reason why it is important to get as many people as possible vaccinated to end the pandemic.
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