Pandemic Perspective #17   July 11, 2020

Is herd immunity a false hope?

Almost since COVID-19 became the leading story of 2020 there has been the hope that herd immunity would bring the pandemic to an end. Not likely. Authorities have flip-flopped on issues such as transmissibility, susceptibility, mask effectiveness, the efficacy of old and new drugs and ventilator therapy. We shouldn’t be surprised if the concept of herd immunity will be tested.

When a new microbe invades a population it sickens some, kills some and infects some who show no apparent symptoms. At a given point there are too few potential victims remaining or they are too widely dispersed so that the infectious agent seems to vanish. It might circulate quietly without causing much notice but can resurface when a new population of susceptible persons arises. The surviving herd is now relatively immune but danger always lurks.

That is not the ideal type of herd immunity because it is only temporary. A more desirable form of herd immunity is based on vaccines. When most of a population has been vaccinated – measles is a good example – a virus that arrives from the outside has no place to go. As long as immunization rates remain high, ideally above ninety percent, herd immunity is permanent and can remain so for generations, unlike that which occurs in natural infections.

Because measles vaccination was so effective by the end of the 20th century the United Sates was declared to be measles-free, a state that quickly changed when increasing numbers of parents refused to allow their children to receive routine childhood vaccinations, including MMR (Measles-Mumps-Rubella). In the past two decades there have been several outbreaks of measles when persons from outside the United States introduced the virus to a population not all of whom had vaccine-induced immunity.

How will the herd react to the coronavirus? The honest answer is that no one knows. In a world without a vaccine it will take years, perhaps decades, before most of the population has been infected, leaving behind so few susceptibles that the virus becomes an annoyance and not a threat, at least temporarily as noted above. As of mid-July there have been fewer than 15 million cases worldwide. Some epidemiologists believe that there are at least ten times that many persons who have encountered the virus but who have been misdiagnosed or missed entirely because they have had few or no symptoms, a total of perhaps 150 million. Among a global population of 7.5 billion that represents only 2 percent who have experienced the virus thus far. The likelihood of a naturally-occurring herd immunity is vanishingly small, especially when government officials are madly scrambling to prevent the development of herd immunity by limiting exposure.

Enter an effective, i.e., fully protective and long-lasting vaccine. Even if some of the more than 150 companies now racing toward this goal are successful, what will it take to provide herd immunity? Again, no one knows. Some vaccine candidates require two doses; some, like the influenza vaccine, may require annual doses. Protecting most of the world’s population with even a single lifetime dose will require a logistical effort unlike any that the world has ever accomplished.

Unless there is an immediate breakthrough with a highly effective drug against the SARS-CoV-2 virus it’s likely that a quarter-million Americans will have died of this disease by the end of 2021. Roughly half will have had conditions such as diseases of the heart, lungs or kidneys; most of those will be the elderly; being overweight and having type 2 diabetes are major risk factors. Perhaps a COVID-19 vaccine will protect individuals in these groups. I certainly hope so.

Unfortunately, herd immunity will not be a factor.