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As the novel coronavirus spreads through much of the globe, the debate over the effectiveness of "herd immunity" continues to swirl in public health and policy circles. So what exactly is this strategy when it comes to fighting an infectious disease?
"Herd immunity is protection in a community from an infectious disease when a large percentage – typically at least 60 percent – become immune," John Whyte, M.D., chief medical officer of WebMD, told Fox News. "They become immune either by getting the infection and getting better – surviving – or by getting vaccinated. Since the majority of people in a community are then immune, there are fewer people who can become infected."
But the voracity of COVID-19 is threatening to shatter health care infrastructures, leading most experts to argue that without a vaccine, it is likely to do more harm than good.
"Herd immunity works best when we have a vaccine as well as a disease which doesn't have serious consequences," Whyte continued. "With no vaccine for coronavirus, it makes herd immunity less effective -- and this disease has serious consequences in some people."
Even when much of the West earlier this month was moving quickly to shutter large gatherings and issue stay-at-home orders, the United Kingdom ignited uproar by maintaining its strategy centered on the herd immunity approach. Rather than imposing firm social distancing measures and ordering public places to close, the leadership sought to initiate progressive restrictions in the hopes that enough of the population would develop such a mild defense that it would curb the spread.
The game plan – against the backdrop of strong condemnation and a rising caseload – by last week had fallen from favor.
"The U.K. initially stated they would rely on herd immunity but quickly did a course correction when modeling showed their hospital system would not be able to address all the serious cases," Whyte explained. "Even if 80 or 90 percent have a mild case, 10 percent is a large number of people for the health system to absorb."
However, other countries – such as the Netherlands – have continued to deploy the theory, despite it being deemed a risky gamble.
"The reality is that in the near future, a large part of the Dutch population will be infected with the virus," Dutch Prime Minister Mark Rutte said in a presidential address last week. "We can slow down the spread of the virus while building controlled group immunity."
He also noted that it could "take months or even longer to build group immunity," and thus, during that time, their objective was to shield people at higher risk, such as the elderly and the immune-compromised, as much as possible.
According to the World Health Organization (WHO), the nature of the coronavirus pandemic requires significantly greater action. Many experts consider the approach to be theoretically sound, but unwise given the absence of a vaccine and the notion that a vaccine will take at least a year to develop.
"The idea behind herd immunity is when enough people or enough of the population becomes infected and builds immunity to say a virus like the COVID-19, its spread naturally stops because not as many people will be able to get it and transmit it to others. Vaccines basically create herd immunity. The herd becomes immune," noted Dr. David Nazarian, a Beverly Hills-based physician, diplomate at the American Board of Internal Medicine and founder of My Concierge MD. "The problem with herd immunity without a vaccine is the overload and stress it will have within the health care system. The reason they are doing social isolation at this time is if too many people become infected at once and a percentage of those require hospitalization, there are not enough healthcare providers, hospital beds and ventilators to provide care for all the sick. This is what we are seeing in Italy now."
In addition, Dr. Stanley Weiss, a professor of Medicine and Epidemiology at Rutgers New Jersey Medical School, underscored that there are still too many unknowns around the novel virus and that the models so far are incomplete, meaning that herd immunity is simply too risky to be relied upon amid the crisis.
"We expect that at some time it will help diminish the spread of COVID-19, based on the assumptions that persons who are infected and recover will have protective immunity," Weiss said. "However, with SARS, data suggested protective immunity was of limited duration. Thus, the precise temporal dynamics may prove very important. For example, there could be recurrent waves. We hope with COVID-19, but do not yet know that they won't become re-infected."
He also pointed out that herd immunity will only be effective on the basis that the virus won't change in ways such that it might evade such protective immunity, and that other similar but different coronaviruses don't emerge.
Nonetheless, experts also point out that herd immunity has played a pivotal role in the history books for abolishing deadly diseases.
"Herd immunity and vaccination has eradicated two highly (infectious) diseases: rinderpest in cattle and smallpox for humans. The verdict is out with coronavirus," said Shaheen Lakhan, vice president of research and development at the neurology and progressive disease-focused The Learning Corp in Boston. "It will depend on how immunogenic our body's response is to the virus, that is, how much protection will antibodies provide against reinfection."
And with a vaccine not likely in the near future, medical professionals are still hoping that herd immunity will be one of the factors that will soon slow the spread.
"Vaccines are an artificial manner of immunity that helps a person deal with the virus, while infections are the most natural and effective way to provide immunity. Both types of immunity last for a very long time," Nazarian added. "A natural form of immunity from actually getting the infection is typically the most effective as it elicits more antibody production and protection for future infections. Eventually, without a vaccine, a large percentage of the population will become infected with the COVID-19 virus, and we will have herd immunity."