What is herd immunity and why does it matter?

In an interview on June 26th on CNN, Dr. Anthony Fauci acknowledged we don’t yet know what the efficacy of a COVID-19 vaccine will be, but that he would “settle” for 70 to 75% effective.  Why that number?  If 70 to 75% of the population is immune to COVID, we would achieve “herd immunity” to quell the outbreak. 

Herd immunity is when a sufficient proportion of the population is immune to an infectious disease, either by vaccination or prior infection, so that spread from person to person is unlikely.  A population with 75% immunity to COVID-19, therefore, would mean that 3 of every 4 people exposed to the virus won’t get sick (and won’t spread the virus any further).  Achieving that level of immunity allows a level of safety for others, who because of age, having a weakened immune system or other medical considerations, cannot be safely or effectively vaccinated.

The challenge, however, is not just to manufacture and distribute enough vaccine to 75% of a population, but for 75% of people to agree to be vaccinated.  A CNN poll in May found that one-third of Americans said they would not try to get vaccinated against COVID, even if the vaccine is widely available and low cost.  Asked if a vaccine with 70-75% efficacy taken by only two-thirds of the population would provide herd immunity, Dr. Fauci answered “No — unlikely.”

Fauci noted that “there is a general anti-science, anti-authority, anti-vaccine feeling among some people in this country — an alarmingly large percentage of people, relatively speaking.”  Given the power or the anti-vaccine movement, “we have a lot of work to do” to educate people on the truth about vaccines.

Sky Pittson, MD, June 29, 2020