Here is the most frequent question I get. It goes something like this. The virus isn’t going away. We can’t stop it without a vaccine and that could be a very long time, and our economy cannot be shut down that long. The only way to slow it down is to achieve herd immunity. We know who is most vulnerable, so why don’t we just keep the elderly and those with serious medical conditions home and get everyone else back to work so that we restore the economy and get to herd immunity?
It is a very insightful question. So, let’s dig in.
First of all, I want to commend the many who have come to the realization that it is very unlikely that this virus is going away anytime soon. Everyone has seen the various models shown on cable and on the internet showing a bell-shaped curve that seems to magically end sometime this summer. These graphs and depictions are terribly misleading. Those really only portray the first wave of this pandemic, and most all experts agree that there will be more waves coming, though it is uncertain how many and what their magnitude will be.
SARS-CoV-2 is an RNA virus and RNA viruses do tend to mutate frequently, and this virus has already mutated a number of times. It is possible that it could mutate and become less infectious or transmissible, and this happened to some degree in the case of the first SARS virus, but this is something we can hope for, but should not anticipate. This would be the only scenario I can think of where the models I referred to above could be right.
Let me also commend the growing number of people who are understanding what herd immunity is. This is the level of immunity in a population required to significantly impair the transmission of the virus. Herd immunity does not mean that a person within the herd (the community) cannot become infected, but it means that the chances are significantly reduced because so many in the herd have immunity that there is no longer efficient transmission of the virus. In other words, there might be isolated cases, but we would be unlikely to see an outbreak of disease in that population.
Herd immunity protects infants who have not yet had the infection and who may be too young to receive a vaccine and it protects those that are high risk, such as those who are elderly and might have waning immunity or those who are immunocompromised and may not be eligible for or protected by a vaccine.
The more contagious a virus is, the higher the level of herd immunity that will be required. For example, the reproduction number (an indicator of how many people someone infected is likely to infect without mitigation in a vulnerable population) is about 6 times higher for measles than it appears to be for COVID. Therefore, herd immunity for measles likely requires that 93-95 percent of the population be immune, either by vaccination or prior infection in order to achieve herd immunity. Mumps has a reproductive number about 5 times that for COVID, and it is estimated that about 90 – 92 percent of a population needs to be immune to prevent spread of mumps to susceptible individuals in that population. For this particular virus, we don’t yet know the level of immunity necessary in a population to slow down the transmission, but projections are in the 60 – 70 percent range.
In Brazil, there was an outbreak of Zika virus. After two years, about 63% of Brazilians had natural immunity (i.e., they had previously been infected and developed immunity to reinfection). This was a high enough percentage of Brazil’s population that the viral spread burned itself out.
With polio on the other hand, a vaccine was developed that was highly effective, and a sufficient number of the world’s population was immunized that polio has essentially been eliminated except from two countries that do not immunize their populations (Afghanistan and Pakistan).
So, now back to the question I get asked. Since a vaccine is not in sight and we know the high-risk populations, why don’t we keep the high-risk individuals at home and get everyone else back to school or work and let’s get to 60 – 70 percent of the population infected so that we have herd immunity?
For illustrative purposes, let’s consider the U.S. a herd. It isn’t because for your purposes, it is the community or population with which you interact and come into contact so it might be quite a bit smaller, but assuming that we want to think about travelling and coming into contact with others at U.S. airports, taking the kids and grandkids to Disneyland, going to sporting events where people from different states are coming together for competitions, visiting other cities and staying in their hotels and going to their restaurants, etc., let’s just look at the United States. (keep in mind that something like 80 million international travelers come to the U.S. each year, so those cities in the U.S. that have disproportionate numbers of international visitors such as Washington D.C., New York City, Los Angeles and Las Vegas may still have outbreaks of disease even if they otherwise have herd immunity, if a sufficient number of these travelers are not also immune. Take for example, McCall, Idaho. Its population can double over the summer months due to visitors and persons with second homes. This could significantly dilute the McCall’s herd immunity if these visitors come from areas that have not achieved herd immunity).
Let’s also assume that we don’t really intend to sentence our seniors to isolation from family, friends and all other contacts for what could be the remainder of their lives until a vaccine does become available. Instead, if we consider the elderly as part of the herd and that we want the herd immunity to give them added protection so that they, too, can return to some semblance of normalcy, though they still may need to exercise precautions, then we will include seniors in the denominator of our total population for achieving 60 – 70 percent immunity. Keep in mind that in our long-term facilities, where many elderly residents have died, it is likely that the exposure of these residents to COVID was through visitors, and more likely, through caregivers who were in contact with others, became infected and then spread the virus to the residents.
The population of the United States is approximately 326.7 million people. Let’s use 2/3rds of the population for the herd immunity threshold. The number of Americans that would have to be infected to reach the herd immunity threshold would be almost 219 million. We don’t really know the case fatality rate for COVID because we do not know how many people have been infected, but even considering asymptomatic and mild infections, most experts seem to think that the mortality rate is at best 0.5 – 1 percent. So, let’s use 0.75 percent. That means that if only 2/3rds of Americans got infected, approximately 1.6 million would die from COVID. Thus far, the U.S. death count from COVID is 65,735 – only about 4 percent of the total deaths that would occur if we were to get 2/3rds of our population infected to achieve herd immunity. That would mean that the U.S. would experience 1,575,932 more deaths from COVID until we reached the herd immunity threshold.
Now there is a whole lot in between mild illness and death. In other words, the price we would pay for herd immunity is more than just loss of life. There are many who would survive, but would endure a prolonged hospital stay, significant morbidity and disability and a prolonged recovery. Keep in mind that while the elderly are more likely to die, 69% of cases, 55% of hospitalizations, 47% of ICU admissions (and generally, it is about half of these who would be on ventilators), and 20% of deaths occur in those under age 65. Many of these affected would be in the prime of their lives.
Not only could just the illness in individuals under age 65 in some cases overwhelm the health care delivery system and result in significant health care costs for society as a whole, but there would be loss of work force productivity due to illness and subsequent disability. For example, of those who require mechanical ventilation, even prior to COVID, these patients not infrequently suffered from traumatic false memories and even post-traumatic stress disorder. This occurs even with the usual support with a ventilator generally being several days. With COVID, those requiring ventilator support often can require it for one to three weeks. We expect that far more will suffer these long-lasting sequelae, resulting in disability, significant ongoing treatment costs and lost productivity at work.
In addition, we have learned that SARS-CoV-2 is not just a respiratory virus. It can cause systemic disease, and in fact, we are seeing a growing number of cases of kidney failure, major strokes and other vascular consequences in young adults in their 30s and 40s. While this is still a minority of young people, the consequences and disability can be significant in these people who otherwise would be in the prime of their lives.
Without a cure for this disease, or at least a treatment that can prevent the disease from becoming severe, we must continue to take infection control measures to slow down the spread of this virus until the virus burns out, we have effective treatments or we have a vaccine. That doesn’t mean that we have to keep the economy on hold. It just means that we have to adapt. Some of these changes will likely need to stay in place after COVID. We need to abandon the handshake. The culture of schools and businesses needs to change to encourage people to stay home when sick. We need to use technology for virtual meetings, for physician visits, for school and working from home, when feasible. And, no doubt many businesses will operate differently, at least for the next year or two. There will likely be more online services, curbside delivery or appointment-based services with screening of both employees and clients. We have to get as many businesses back open as soon as possible and as safely as possible.
For the reasons above, the answer cannot be full-speed ahead with the economy in an attempt to achieve herd immunity. But, at the same time, the answer also cannot be to shut down the economy for a couple of years to achieve herd immunity through vaccination. We have to strike the right balance between slowing the spread of the virus to a manageable level that does not overwhelm our health care infrastructure or result in more mortality and morbidity than necessary, but yet getting businesses back open and people back to work because there are also many excess health consequences for people who are unemployed, uninsured and unable to access routine health care.
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