While the argument rages on about when and how to reopen the American economy, many experts think one statistic may be more important than the case count or number of deaths. The number of recovered COVID-19 patients may be key to safely reopening society, according to a new scientific model.
Despite the claims to the contrary on the part of some, those who want to return to some semblance of “normal life” aren’t interested in putting their fellow citizens – or their loved ones – at risk. Minimizing new infections, reducing hospitalization and death rates further and preventing a “second wave” should all be part of the equation.
But what about “shield immunity?” This refers to the presumed immunity of people who have been infected by, and recovered from, COVID-19. It may be possible for some of these people to substitute for those who are in susceptible or high infection risk categories in some high-contact occupations like healthcare.
The belief is that this immunity provides a shield against short-term reinfection. Those with this immunity would be able to tolerate interactions with infected and susceptible people. This has the potential to lower overall infection rates as social and professional interactions are allowed to expand.
Recent research by organizations like USC and Stanford University, among others, strongly indicates that the percentage of the American population already infected by the coronavirus is as much as 100 times higher than originally reported. In this case, bad news is good news.
A higher infection rate means the mortality risk is statistically negligible. While case fatality rates remain somewhat higher than seasonal flu, the mortality risk for individuals is extremely low. The exception to this is among adults over 70, especially those with existing comorbidities and certain other people with comorbidities, such as those who are immuno-compromised.
This, combined with the concept of shield immunity, may be good news to public officials trying to restart the economy and for people who’d like to be social again, at least to some degree. New scientific modeling of how the virus behaves indicates that an intervention strategy involving shield immunity may cut the risk associated with increased human interaction as the economy reopens and begins to expand.
Two caveats exist. One involves identifying who has antibodies to SARS-CoV-2. To accomplish this on the scale required by the new model, a level of reliable serological testing is required that the US is nowhere near achieving at this stage. The second revolves around the question of immunity duration. We don’t know how long recovered patients are immune to SARS-CoV-2. We do know, however, that survivors of Middle East Respiratory Syndrome (MERS) had immunity for about three years, while those who survived SARS, the precursor to the current pandemic virus, had persistent antibodies for about two years.
“Our model describes ways in which serological tests used to identify individuals who have been infected by and recovered from COVID-19 could help both reduce future transmission and foster increased economic engagement,” said Joshua Weitz, professor in the School of Biological Sciences and founding director of the Interdisciplinary Ph.D. in Quantitative Biosciences at the Georgia Institute of Technology. “The idea is to think in advance about how identifying recovered individuals could help serve the collective good, using information collected on neutralizing antibodies in new ways.
The research team, comprised of scientists from Princeton University, McMaster University and Georgia Tech, used a computer model of COVID-19 epidemiological dynamics. It was built on what’s called a SEIR, or susceptible-exposed-infectious-recovered framework. This allows researchers to model the flow of people in the four states of the disease as they move into and through society and economic activity. The paper describing the model was published in the journal Nature Medicine
In a population of 10 million citizens, for example, the model predicts that in a worst-case transmission scenario, implementation of an intermediate shielding strategy could help reduce deaths from 71,000 to 58,000, while an enhanced shielding plan could cut deaths from 71,000 to 20,000. The model also suggests that shielding could enhance the effects of social distancing strategies that may remain in place once higher levels of economic activity resume.
There have been significant advances in both the quality and distribution of antibody tests in the US in recent weeks, but there remains much work to be done in that area. Another issue is that antibody tests are not 100% specific, meaning false positives are a possibility.
If testing is targeted in groups known to have increased levels of exposure, we can increase the predictive value of even imperfect tests. While nearly everyone has heard of PCR, or polymerase chain reaction testing, serological antibody tests differ in that they look for recovered people, rather than people who are currently infected.
Currently, the number of recovered individuals with immunity is believed to be small. That assumption is made, however, on the limited amount of serological antibody testing actually done. If we take the new figures on presumed infections across the country and plug them into the equation we’re currently using to determine who should be tested, the potential shield immunity pool increases greatly.
These individuals could substitute for susceptible people in industries like food service, retail, construction, nursing homes, emergency medical services and others. While some jobs can’t be easily substituted for, some are suitable candidates with minimal training. Placing those with immunity in even some of these positions can significantly reduce the population-wide infection risk.
“We want to think about serology as an intervention,” Weitz said. “Finding out who is immune to the coronavirus could make a big difference in trying to reduce the risk to people who would be vulnerable by interacting with someone who could pass on the disease.”
Healthcare workers are the most logical group on whom to perform serological antibody testing first. Their exposure to infected people makes them more likely to have been infected themselves, according to Weitz. Given the growing number of people who seem to have had the virus either asymptomatically or with minimal symptoms, it’s highly likely that a lot of people have recovered without realizing they had COVID-19. That means a much bigger pool of recovered, antibody-positive people.
“There may be a deeper pool of individuals who can help within their own fields and other fields of specialization to reduce transmission,” Weitz said. “The reality is that people within high-contact jobs probably are likely to have a higher incidence of infection than other groups.”
There are, of course, privacy risks and issues. Recovered individuals can’t be compelled to reveal their status by becoming part of a substitution program. Many, however, would likely accept the additional risk for the greater good.
“What this model says is that if we could identify individuals who are immune, there is a chance that some individuals would not have to reduce their level of interaction with others because that interaction would be less risky,” he added. “Rather than trying to keep reducing interactions, which is helpful for reducing transmission but bad for what it does for the economy, we might be able to maintain interactions while reducing the risk, combined with other mitigation approaches.”
Many still believe that a vaccine is the only way to boost immunity levels beyond 60 percent. But that may or may not be the case. No vaccine was ever developed for SARS. Yet case counts fell to levels considered statistically negligible. Factor in the experience of Sweden, where no lockdowns were instituted and case and death counts per capita matched some countries with lower-end coronavirus experiences. Some of the newest reports out of Stockholm state that over 25% of the people there have immunity.
It makes sense then, that Weitz and company may be on to something with the idea that shield immunity may be a link to getting our world reopened.
“We don’t have a silver bullet,” he said. “Until we have a vaccine, we will have to use a combination of strategies to control COVID-19, and shield immunity is potentially one of them.”
We’ve much to learn about SARS-CoV-2. Thankfully, some of the brightest people on the planet are on the job, so to speak. Until they solve this, stay safe, be healthy and…
Keep the faith and keep after it!
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Journal Reference – https://www.nature.com/articles/s41591-020-0895-3