The mass vaccination site of COVID-19 at the Lumen Field Events Center in Seattle. (Photo courtesy of Governor Inslee / Twitter)
As more and more people are vaccinated against COVID-19, the concept of herd immunity continues to emerge in conversations. Some health officials have previously expressed that true collective immunity can be difficult to achieve, and Dr. Jeff Duchin, health officer at Public Health – Seattle and King County, agrees.
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Herd immunity, Duchin explained, is a concept that comes from animal health. It essentially means that the population has sufficient immunity so that a virus does not spread effectively.
“If a person is infected, the people around him are so immune due to natural infection or vaccination that the virus cannot be transmitted to the population,” he said.
So far, something like collective immunity has been achieved for measles in the United States, although Duchin notes that there are pockets of unvaccinated people where measles can still spread. Until recently, it could not spread widely because many people were vaccinated against it. In addition, the measles vaccine, says Duchin, offers high-level, long-term protection.
Since the beginning of the COVID-19 pandemic, several people have stopped taking their vaccines, which Duchin noted will be a real problem going forward. He encouraged anyone who died – their children or herself – to get vaccines routinely, especially for measles, mumps and rubella.
As for obtaining collective immunity for COVID-19, Dr. Duchin believes that it is “probably unlikely” that there will be sufficient immunity in the population to completely stop the spread of the virus.
“But I think we can achieve a very significant level of population immunity so that our community is protected from serious outcomes, hospitalization and death, but it is probably not able to completely prevent the transmission of the virus from person to person.” he said.
There are a few reasons why he believes this to be the case, starting with the fact that there is currently no vaccine for children.
“You cannot get herd immunity when 20% of the population is not vaccinated,” he said.
“Second, we would need very high levels of immunization coverage to achieve collective immunity, and that would mean for the entire population,” he added.
Initially, it was thought that herd immunity could be achieved by 70%, but because of the more communicable variants, Duchin says that even higher levels – perhaps 80% or more – are likely to need to be vaccinated to prevent any transmission succeed and achieve true herd immunity.
“The other complicating factor is that, although vaccines are tremendously effective in preventing serious illnesses, hospitalization and even symptomatic illnesses, we don’t know exactly how effective they will be in preventing transmission,” said Duchin. “The initial data suggests that it will be very effective, but not 100%. This is great news because it will decrease the risk, but it will not eliminate it. “
Duchin also pointed out that the duration of protection is still unknown for COVID-19 vaccines.
“It is possible that the population needs to be re-immunized to maintain immunity,” he said. “So it is a very significant logistical challenge to continue to have a very high proportion of the entire population with a high level of vaccination coverage on an ongoing basis, forever.”
Finally, as the COVID-19 virus and its variants evolve, Duchin warns that they may become less susceptible to vaccine-induced protection, perhaps requiring that new vaccines be produced and modified. Duchin compared this situation with flu vaccines, where the virus mutates and new vaccines are developed to “meet the challenge of evolving viruses”.
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“So, for a number of reasons, I think that real herd immunity, that is, the absolute blocking of transmission in the population, is probably not going to happen,” he said. “But I think what is very realistic is that we will be able to achieve a sufficient level of immunity through vaccination to protect our population from the most serious effects of COVID-19 on health, hospitalization and death. And luckily, if people are vaccinated in large numbers, drastically reduce the number of cases. “
“It is somewhat analogous to the flu situation, where we have vaccines every year that can prevent morbidity, mortality and serious hospitalizations, but milder cases occur, transmission occurs and we have to live in balance with a virus going forward”, he added. “I think that high-level population immunity is probably more realistic than real herd immunity.”