LOS ANGELES - The term “herd immunity” has spread in conversation about the novel coronavirus almost as a beacon of hope in the fight against the deadly disease — until, at least, a vaccine is created. But researchers continue to find evidence that the concept of immunity from widespread infection may not apply to COVID-19.
A study published on July 11 by researchers at King’s College London found that antibodies detected in the human body which fight the coronavirus declined after just a few weeks, leaving the possibility of herd immunity out of the question.
Epidemiologists define the herd immunity threshold for a given virus as the percentage of the population that must be immune to ensure that its introduction will not cause an outbreak. If enough people are immune, an infected person will likely come into contact only with people who are already immune rather than spreading the virus to someone who is susceptible.
The implications of such a study engender the possibility that future waves of the novel coronavirus could return in seasons.
Researchers from King’s College analyzed data from 96 patients and health care workers to understand the development of antibodies over the course of three months.
They found that 60% of the participants built up a “potent” antibody response while only 17% retained that potency at the end of the three-month study period.
This study seems to confirm findings from other researchers who have urged that the lack of potential for herd immunity means that social distancing measures are the best chance against the virus until a vaccine arrives.
Proactive mitigation strategies like social distancing and wearing masks can flatten the curve of the virus by reducing the rate that active infections generate new cases. This delays the point at which herd immunity is reached and also reduces casualties, which should be the goal of any response strategy, according to experts.
A separate nationwide population-based study published by researchers in Spain last week in the medical journal “The Lancet” found that despite the high impact of COVID-19 on the country, antibody “prevalence estimates remain low and are clearly insufficient to provide herd immunity.”
Researchers in Madrid tested 61,075 participants from April 27 to May 11 and found that only 5% maintained antibodies developed from the virus.
Additionally, only 14% of people who tested positive for coronavirus antibodies in the first round of the study’s testing eventually tested negative in future tests carried out only weeks later.
"These results emphasize the need for maintaining public health measures to avoid a new epidemic wave," the authors wrote. "Social distance measures and efforts to identify and isolate new cases and their contacts are imperative for future epidemic control."
"In light of these findings, any proposed approach to achieve herd immunity through natural infection is not only highly unethical, but also unachievable," they added.
Herd immunity is usually discussed in the context of vaccination. For example, if 90% of the population (the herd) has received a chickenpox vaccine, the remaining 10% (often including people who cannot become vaccinated, like babies and the immunocompromised) will be protected from the introduction of a single person with chickenpox.
But herd immunity from SARS-CoV-2 is different in several ways:
1) We do not have a vaccine. As biologist Carl Bergstrom and biostatistician Natalie Dean pointed out in a New York Times op-ed in May, without a widely available vaccine, most of the population – 60%-85% by some estimates – must become infected to reach herd immunity, and the virus’s high mortality rate means millions would die.
2) The virus is not currently contained. If herd immunity is reached during an ongoing pandemic, the high number of infected people will continue to spread the virus and ultimately many more people than the herd immunity threshold will become infected – likely over 90% of the population.
3) The people most vulnerable are not evenly spread across the population. Groups that have not been mixing with the “herd” will remain vulnerable even after the herd immunity threshold is reached.
The Associated Press contributed to this report.