Health officials around the world are rushing to vaccinate enough people to prevent the spread of COVID-19, but what qualifies as “enough” is still an open question.
The objective is to obtain “herd immunity”, which is when a sufficient number of people have immunity, either through vaccination or past infection, to prevent uncontrolled spread.
Herd immunity does not make any person immune, and outbreaks can still arise. This means that a virus is no longer jumping easily from person to person, helping to protect those who are still vulnerable to contracting it.
No one is sure what the herd’s immunity limit for coronavirus is, although many experts say it is 70% or more. And the emergence of variants further complicates the picture.
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Here’s what is known about the virus and collective immunity.
HOW IS THE IMMUNITY LIMIT OF THE FLOCK CALCUED?
It is a formula based on how contagious a virus is – or how many people catch the virus from an infected person, on average.
But the calculation offers only a broad target for when there may be a big drop in the spread. The figure may also vary by region.
“It’s not 64.9 it’s terrible and 70.1 is fantastic,” said Dr. Walter Orenstein, an infectious disease specialist at Emory University.
Orenstein notes that vaccination levels and other factors that affect spread can differ even within a city.
HOW DO WE KNOW THAT WE ACHIEVE FLOCK IMMUNITY?
Proof that we are approaching collective immunity would be a “disruption in the transmission chain,” said Ashley St. John, who studies the immune system at Duke-NUS Medical School in Singapore.
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But don’t wait for any big statement that we have reached that milestone.
To determine whether restrictions should be relaxed, health officials will be watching trends in infection and hospitalization as vaccines are implemented. And these decisions are likely to start long before the ideal herd immunity limit is reached, although they are gradual and vary by region.
In India, for example, scientists believe that more people will need to be protected in densely populated cities, where the virus spreads faster, than in its vast field.
India plans to look for antibodies in people across the country to find out what percentage of its nearly 1.4 billion people have already been infected, said Dr. Jayaprakash Muliyil, who is advising the government on virus surveillance.
The vaccine’s effectiveness also plays a role. Fewer people need to be vaccinated to obtain collective immunity if vaccines are more effective.
HOW DO CORONAVIRUS VARIABLES AFFECT THE FLOCK IMMUNITY?
It depends on the protection that a past infection or vaccination offers against the variant.
If vaccines were shown to be less effective against a variant, it would be necessary to vaccinate an even larger portion of the population or update existing vaccines to make them more effective, Orenstein said.
So far, it appears that the photos provide at least some protection against the most worrying variants. But scientists are still studying the situation and are concerned about new mutations.
The variants highlighted the importance of vaccinating people as quickly as possible. Delaying transmission is critical, as viruses can mutate when infecting people.
DOES HERD IMMUNITY HAVE TO BE GLOBAL?
Global herd immunity is ideal, but unlikely.
Rich nations have reserved most of the vaccines that will be manufactured this year. In the United States, for example, officials said that enough people could be vaccinated by autumn to start returning to normal.
But many poorer countries are likely to have to wait longer. That is why the World Health Organization has warned that global herd immunity is unlikely to be achieved this year.
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Differences in vaccination levels between countries are also the reason why many experts believe the virus will never be completely eliminated.
CAN THE IMMUNITY OF THE FLOCK WEAR?
It is not known how long immunity lasts, after vaccination or an infection, although experts believe it should take at least several months.
Still, reinforcement shots may be needed in the future. And while the current COVID-19 vaccines are expected to work on the variants identified in the UK and elsewhere, it is possible that the virus will mutate sufficiently over time for the vaccines to be updated.
Mutations in influenza viruses, for example, are the reason why we get flu shots every year. But experts note that coronaviruses generally don’t mutate as easily.
WHAT IF COVID-19 VACCINES DO NOT AVOID INFECTION?
The COVID-19 vaccines launched now appear to be very effective in preventing disease in people. We still don’t know how effective they are in completely preventing infection, but they should help to significantly reduce the spread of the virus.
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That’s because vaccines prevent the virus from multiplying in your body. So even if you become infected after vaccination, your body should release less virus and for a shorter period, said Deborah Fuller, a vaccine expert at the University of Washington.
It is another reason why vaccinating as many people as possible is the key to ending the pandemic.