Fifty-five years ago, as I became interested in smallpox, I was told repeatedly that “herd immunity” for the infectious disease would be achieved when 80% of a population was immune, either from vaccination or surviving the infection.
That turned out not to be the case then, and a herd immunity approach is likely to be inadequate now in the battle against the novel coronavirus.
The current discussions of herd immunity often concern hypothetical possibilities and are not based on evidence. Even when science-based discourse is presented, it is often one dimensional.
The discussions provide an immune percentage that supposedly gives the point at which natural transmission will cease. The crucial feature is how many susceptible people are in a given area. No herd immunity figure is meaningful without population density, plus information on the frequency and types of contacts between people.
The campaign to eradicate smallpox provided real-life experience demonstrating the inadequacy of herd immunity as a concept or valid target. The vaccine was highly effective, inexpensive and easily administered, with more than 95% of those vaccinated developing decades-long or even lifetime immunity.
Reaching that level seemed an achievable goal. Successful smallpox immunization does not require a laboratory test. Simply looking at the vaccination site a week or two later will confirm a local infection, which, based on many studies, confirmed the person had achieved immunity to smallpox.
The World Health Organization approach to smallpox eradication in 1967 was to have one or two mass campaigns to attain herd immunity in large geographic areas, followed by a second strategy of outbreak containment in any remaining areas of infection.
In 1967, we vaccinated the city of Abakaliki in Nigeria. Subsequent evaluation confirmed that more than 90% of the population had been successfully vaccinated, exceeding the herd immunity anticipated threshold. Yet weeks later the city suffered a smallpox outbreak. Even 90% coverage with a highly protective vaccine proved insufficient.
About the same time, a temporary shortage of vaccine caused us to limit vaccinations in one outbreak. We identified contacts of the smallpox cases and were so successful that the outbreak ended with only 7% of the population vaccinated. Dropping the idea of herd immunity as the target, an approach ingrained in public health thinking at the time, allowed a fresh and innovative approach. The absence of smallpox was now the goal.
This experience suggested WHO could simply eliminate the first half of the strategy (mass vaccination to achieve herd immunity) and go directly to the second half of the strategy, attacking the virus directly using search and containment tactics.
Learn from work in India
In 1973, this approach was tried in India. For more than a century, the objective of smallpox programs in India had been to try for herd immunity. Experts from around the world would gather to review plans for each new campaign, and the advice was always concentrated on how to achieve 80% herd immunity.
At the end of each campaign, the evaluators reached the same conclusion: Smallpox rates had not declined, and the 80% target had not been achieved. The penultimate campaign plans were reviewed by leading global experts, who recommended changing the objective to 100% vaccination coverage. How such outside experts could suggest that the failure to achieve 80% coverage required an even higher unreachable goal is a mystery.
The African approach was inaugurated in India with a major surprise. A village-by-village search for cases was started during the low smallpox transmission season, to get search experience. In six days, searchers found 10,000 new cases of smallpox in only two states. That overwhelmed the response system.
Six months were required to adapt the search and containment strategy to Indian conditions, but in May 1975, 12 months after the system was in place, the entirety of India had gone from the highest rates of smallpox in decades to zero, one of the most gratifying years in global health history.
Vaccines are only part of the solution
Those who advocate a herd immunity approach for controlling the coronavirus have not studied the lessons of the past. It is not likely that coronavirus vaccines will be as effective as smallpox vaccine. It is not likely that the duration of immunity will be as solid and lengthy as with smallpox. It will not be as easy to confirm vaccine immunity as was the case with smallpox.
Herd immunity turned out to be an ineffective strategy even for smallpox. Suggesting such a losing strategy for coronavirus is to ask for more suffering, more deaths and more social disruption.
Such a program would prove wrong those who said our current program couldn’t get worse.
Dr. William Foege, a professor of international health at Emory University, worked in the successful campaign to eradicate smallpox. He is a former director of the Centers for Disease Control and Prevention.
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This article originally appeared on USA TODAY: 'Herd immunity' to fight COVID-19 is losing strategy: ex-CDC director