The Importance of Herd Immunity

As physicians interested in solving COVID-19, we talk a lot about herd immunity. That’s where a minimum number of people, usually cited as 70 – 80%, give or take, get vaccinated and are able to collectively bring the disease risk down for everyone.

Lately, I have heard the same question a few different times. “Why is it important to reach herd immunity? If all of the older people, who are most at risk, are vaccinated, isn’t that good enough?” It’s not. Let me tell you why.

Young people do get sick. Some young people experience moderate initial symptoms and go on to have long-term effects, like diminished exercise capacity. Some experience symptoms that keep them out of school for many days to weeks. Others experience severe disease requiring hospitalization.

Illness, even among young people, causes a ripple effect. Unvaccinated people who get sick have to stay out of work or school. This may have a significant negative impact to them as individuals, but society suffers too. Businesses struggle to keep enough staff to stay open, and learning for everyone in the class grinds to a slow pace due to absenteeism.

Unvaccinated people are the virus’s playground. Unvaccinated people spread the disease to others at a much higher rate than vaccinated people and are a greater source of viral mutation and variant development.

Vaccination is highly effective but not 100% preventive. Vaccines are amazingly powerful and provide incredible protection. That’s why vaccinated people can socialize freely with other vaccinated people with little to no risk. But the vaccines are not 100% effective. A population with a high percentage of unvaccinated individuals will have a relatively high prevalence of circulating virus. That means that vaccinated people who interact with those who are not vaccinated will still get sick occasionally.

Think of it terms of statistical chance, like drawing a number five out of a jar filled with 20 other numbers. Let’s say you and most other people are vaccinated. Then prevalence of the virus in your community will be low. You will only occasionally bump into the virus, maybe a few times per year. And you are protected by the vaccine, so the chances of getting COVID is extremely low, just as it is unlikely you would choose the number 5 out of the jar if you picked a number only a few times each year.

However, let’s now say you and only half of the population is vaccinated. Then prevalence of the virus in your community will be moderately high. You will bump into the virus often, at least several times per week. Even with a 90 – 95% effective vaccine, your chances of getting COVID is high. Because you are encountering the virus often, you would eventually get COVID, just as you would eventually choose the number 5 out of the jar if you picked over and over again.

One good caveat to mention is that if you are vaccinated and do have the unfortunate chance of contracting the virus, it is likely to be only mild illness. The vaccine protects you from becoming severely sick.

Immunity doesn’t last forever. Immunity in those with prior infections and those vaccinated will wane over time. While we will have booster shots, it will be a while longer before we know what frequency will be necessary. Herd immunity will help bridge the gap between when our vaccine or naturally occurring immunity begins to wane and when boosters are ready.

That’s why “good enough” means reaching a vaccination rate of 70% or higher, a minimum percentage of vaccination necessary to keep the virus prevalence low. If we can get there, COVID-19 will have an effect on the population similar to influenza. If we can’t, the virus will wax and wane, spiking to significant levels in the winter months, and disrupt life for years to come.

Trey Dobson, MD, is an Emergency Physician at Dartmouth-Hitchcock Health and the chief medical officer at Southwestern Vermont Medical Center in Bennington, VT.