Herd immunity requires larger number of vaccinations

Dear Doctor: Ever since the pandemic started, people have been talking about “herd immunity” and how they see it happening. Sometimes it doesn’t even seem like they’re talking about the same idea. What is herd immunity? Has it ever happened? Will we reach it with the novel coronavirus?

Dear Reader: Herd immunity is the protective effect that occurs when enough of a population is resistant to a contagious disease that it becomes difficult for the infection to spread. This widespread immunity helps protect those who remain vulnerable to infection, such as infants and children too young to get the vaccine, as well as adults who may be immunocompromised.

The protective effect of herd immunity is usually arrived at via the process of vaccination. Perhaps the most successful example is smallpox, a contagious and disfiguring disease that once killed one-third of those who became infected. Another great example is polio, which was once one of the most-feared illnesses in the U.S. Caused by the poliovirus, it’s a life-threatening disease that left at least 15,000 people paralyzed each year, many of them children.

Herd immunity to both smallpox and polio was reached through nationwide -- and worldwide -- vaccination campaigns. The last natural outbreak of smallpox in the United States occurred in 1949, and the disease was declared eradicated worldwide in 1980. No cases of polio have originated in the U.S. since 1979. However, the disease is still a threat in some countries, which is why the polio vaccine continues to be recommended. While it’s theoretically possible to reach herd immunity through antibodies conferred when enough people have become sick and recovered from a disease, this takes a steep toll in terms of illness, suffering and lives lost.

Eve Glazier, M.D. and Elizabeth Ko, M.D

When it comes to reaching herd immunity to the coronavirus that causes COVID-19, we face several complications. One is the fact that the contagiousness of a virus impacts the percentage of the population that must become resistant to it in order to stop its spread. It was originally hoped that immunity to the coronavirus by 60% to 70% of the population would be sufficient. However, the continued emergence of highly contagious variants of the virus has raised that estimate to at least 80%, and perhaps higher. This makes both the pace and the scope of vaccination programs all the more important.

Unfortunately, due to uneven vaccination availability worldwide -- and persistent vaccine hesitancy -- that goal isn’t being met. Half of all adults in the U.S. have now received at least one dose of a vaccine, but those rates have begun to slip. Also, some people are skipping the second dose of the two-dose regimen. Rather than herd immunity, we’re faced with the prospect of COVID-19 becoming a manageable illness.

Thanks to the vaccinations that have taken place, as well as advances in treatment of active disease, we can expect future outbreaks of COVID-19 to be smaller and less deadly, according to officials from the Centers for Disease Control and Prevention. And as vaccine development and vaccination campaigns continue, herd immunity sometime in the future remains a possibility.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o UCLA Health Sciences Media Relations, 10880 Wilshire Blvd., Suite 1450, Los Angeles, CA, 90024. Owing to the volume of mail, personal replies cannot be provided.)


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