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As more and more people get vaccinated against COVID-19, the concept of herd immunity keeps coming up in conversation. Some health officials have previously expressed that true herd immunity may be hard to reach, and Dr. Jeff Duchin, health officer with Public Health — Seattle & King County, agrees.

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Herd immunity, , is a concept that comes from animal health. It essentially means that the population has so much immunity that a virus cannot spread effectively.

鈥淚f one person becomes infected, the surrounding people are so highly immune either due to natural infection or vaccination that the virus can鈥檛 transmit in the population,鈥� he said.

So far, something like herd immunity has been achieved for measles in the United States, though Duchin notes that there are pockets of unvaccinated people where measles can still spread. Until recently, it couldn鈥檛 spread widely because so many people are vaccinated against it. Additionally, the measles vaccine, Duchin says, offers long-standing, high-level protection.

Since the start of the COVID-19 pandemic, a number of people are skipping their vaccinations, which Duchin noted will be a real issue going forward. He encouraged anyone who has lapsed 鈥� their children or themselves 鈥� to catch up on routine vaccinations, particularly for measles, mumps, and rubella.

As far as achieving herd immunity for COVID-19, Dr. Duchin thinks it鈥檚 鈥減robably unlikely鈥� that there will be sufficient immunity in the population to completely stop the spread of the virus.

鈥淏ut I do think we can achieve a very meaningful level of population level immunity so that our community will be protected from severe outcomes, hospitalization, and death, but not likely be able to completely stop the virus from transmitting from person to person,鈥� he said.

There are a few reasons why he believes that鈥檚 the case, starting with the fact that there鈥檚 currently no vaccine for children.

鈥淵ou can鈥檛 achieve herd immunity when 20% of the population is not vaccinated,鈥� he said.

鈥淪econdly, we would need very high levels of immunization coverage to achieve herd immunity, and that would mean of the entire population,鈥� he added.

Initially, it was thought herd immunity could be reached at 70%, but because of the more transmissible variants, Duchin says it鈥檚 likely that even higher levels — perhaps 80% or more — would need to be vaccinated to prevent any transmission from being successful and reach true herd immunity.

鈥淭he other complicating factor is that although vaccines are tremendously effective at preventing serious illness, and hospitalization, and symptomatic disease even, we don鈥檛 know exactly how effective they will all be at preventing transmission,鈥� Duchin said. 鈥淚nitial data suggest they鈥檒l be very effective, but not 100%. So that鈥檚 great news because that will decrease the risk, but it won鈥檛 eliminate it.鈥�

Duchin also pointed out that the duration of protection is still unknown for the COVID-19 vaccines.

鈥淚t鈥檚 possible that the population may need to be re-immunized in order to maintain immunity,鈥� he said. 鈥淪o that 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.鈥�

Finally, as the COVID-19 virus and variants evolve, Duchin warns that they could become less susceptible to vaccine induced protection, perhaps requiring new vaccines to be produced and modified. Duchin compared this situation to the influenza vaccines, where the virus mutates and new vaccines are developed to “meet the challenge of the newly evolving viruses.”

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鈥淪o for a number of reasons, I think that true herd immunity, meaning the absolute blockade of transmission in the population, is probably not going to happen,” he said. “But I think that what is very realistic is that we will be able to achieve sufficient level of immunity through vaccination to protect our population from the most serious health effects of COVID-19, hospitalization, and death. And, hopefully, if people become vaccinated in great enough numbers, drive down the number of cases dramatically.鈥�

鈥淚t鈥檚 somewhat analogous to the influenza situation where we have vaccines each year that can prevent serious morbidity, mortality, prevent hospitalizations, but milder cases do occur, transmission does occur, and we have to live in equilibrium with a virus going forward,” he added. “I think that high-level population immunity is probably more realistic than true herd immunity.鈥�