Quantifying Our Hunches: A Review of Two Recent COVID Articles

Last spring, as scientists rushed to understand a novel virus and describe the disease it causes, I wrote reviews of several key articles in scientific journals (the first review being here and the second being here), in hopes that it would help us nerds make sensible decisions to protect ourselves. Now we are getting through the initial unknown phase of the SARS-CoV-2 pandemic and moving to a normalization of life beyond the panic. There have been a couple recent journal articles that I think might help us predict the effect COVID-19 will continue to have on our lives.

By now, many of us recognize a SARS-CoV-2 virus. CDC

SARS-COV-2 Infection Rates

Source: “SARS-CoV-2 Infection Rates” (Victoria Jane Hall, Sarah Foulkes, et al., Lancet 2021; 397: 1459–69)

The full name of this article is also descriptive: “SARS-CoV-2 infection rates of antibody-positive compared with antibody-negative health-care workers in England: A large, multicenter, prospective cohort study (SIREN).” This means that the authors followed health care workers from all over England. Voluntary enrollment began last spring, eventually reaching about 23,000 participants. Each person who signed up for the study was tested on entry for SARS-CoV-2 antibodies, sorting them into “antibody-positive” and “antibody-negative” groups.

Their “SARS-CoV-2 infection rates” were determined by testing all of the participants every two weeks with nasal swabs (PCR tests) and screening for symptoms of COVID-19. They also received blood antibody tests every four weeks. This report is only a small part of the SIREN study—for SARS Immunity and Reinfection Evaluation (NHS) study—which is still ongoing. This paper uses data from June to December of 2020.

The “antibody-positive” group included everyone who had laboratory documentation of a COVID-19 infection by nasal swab and anyone whose blood tested positive for antibodies at their entry into the study. There were 8,278 health-care workers in this group. There were 14,777 health-are workers who had no evidence of a prior COVID-19 infection and no antibodies—the “antibody-negative” group. Both groups consisted mostly of nurses (45%) and doctors (12%).

As expected, there were fewer infections in the positive group. Only 155 people (2.2%) who had already had COVID-19 developed a second infection during the seven months of the study, and only about half of them had any symptoms. But 1,859 people (12.5%) in the negative group got infections, with 80% of those developing symptoms.

This is a massive amount of work to, essentially, prove something that we already were pretty sure of—that COVID-19 infection mostly protects against recurrent COVID-19. It also proves that reinfections, at least during the first year, are not as severe. As a bonus, a new strain of SARS-CoV-2 (B.1.1.7) became prevalent during the study, and it did not noticeably alter infection rates.

This is the first time I’ve seen the need for a study to prove that getting a virus generates immunity to it. Usually vaccine makers have to prove that their shots are as good as natural immunity. The conclusion of the study is that COVID-19 infection “provides a high degree of immunity to repeat infection,” and the protection is as good as that generated by any of the currently licensed vaccines.

Limiting Indoor Transmission

Source: “A Guideline to Limit Indoor Airborne Transmission of COVID-19” (Martin Z. Bazant and John W. M. Bush, PNAS, April 27, 2021; 118 [17])

This paper has gotten a fair amount of publicity, since author Martin Bazant, perhaps unwisely, told reporters at CNBC that “you’re as safe at 6 feet as you are at 60 feet.” This has been misinterpreted in a variety of news outlets, in a variety of ways, mostly criticizing public health social distancing and masking policies.

The paper itself is carefully done and serves to quantify and validate things that have been suspected about the spread of COVID-19 all along. We all have been told that SARS-CoV-2 travels from person to person in “droplets” created by coughing and sneezing. Large droplets rarely travel more than six feet before falling out of the air, thus forming the basis for six-feet social distancing.

In fact, respiratory droplets vary in size from visible to microscopic “aerosol” particles. These smaller particles are generated just by breathing, and can stay airborne, float around for hours, and disperse widely. It has been known for some time that the virus can survive and travel in aerosol form, but this inconvenient truth has not been used to inform much public health policy. The new CDC guidelines published on May 7 finally admit to the possibility of airborne spread.

The authors are professors at MIT; Bazant teaches chemical engineering, and Bush teaches applied mathematics. The paper is in the peer-reviewed journal Proceedings of the National Academy of Science (PNAS), published on April 13, 2021. Their paper uses the known physics of small airborne particles to calculate the rate and distance of spread of aerosolized SARS-CoV-2 particles as they diffuse into an enclosed room.

(I am neither an engineer nor a mathematician, but I can say that their methods make sense to me as a physician informed in epidemiology and virology. If you wish to question their engineering/mathematical process, there is an entire second paper which rigorously details the origins of the formulas used in their calculations.)

The authors use formulas for aerosol diffusion to determine the time it would take for a single infected person to fill a closed room with a potentially infectious concentration of SARS-CoV-2 particles. They call this Cumulative Exposure Time (CET). They show that, given enough time, an entire room can be filled with tiny, potentially virus-containing particles, if the space is small enough and the air is not being replaced or filtered. They found that the concentration of particles in the ambient air can easily climb high enough to infect any other person in the room, at six or sixty feet away.

They calculate that a small closed space, like a hospital room, can reach an infectious concentration of virus in as little as fifteen minutes. A larger, well-ventilated room, such as a classroom, might take 8 to 10 hours before there is enough virus to pose a risk. This explains why the virus spreads so easily in tiny nursing home rooms, and does not spread as much in schools. Well-ventilated classrooms do not reach significant concentrations in an eight-hour day, and they clear while they are empty overnight.

This is cigarette smoke, which also diffuses as an aerosol. CCO Public Domain via Mayo Clinic News Network

The authors also encourage continued mask wearing. An infected person is exhaling a “plume” of droplets and aerosol directly in front of them, so concentration of virus will be much higher there. It is fairly obvious that masks redirect this “plume,” making the immediate (within six feet) area safer. Masks do not completely filter aerosol particles, but they do redirect them. This more than doubles the CET, increasing the time a single infectious person can be in a room before endangering others.

The mathematical framework provided can theoretically be used to predict exactly the length of time it is safe for a group of people, any one of whom may be infectious, to stay in an indoor space. Even without doing the math, though, it is clear that spending a long time in an enclosed space with poor ventilation increases your risk, even if you manage to stay six feet away from others. Eventually you end up swapping air with everyone in the room.

Living in Our New COVID World

A few useful conclusions can be made from these two papers. Here are some predictions.

I’m happy that we now have definite evidence that SARS-CoV-2 acts just like any other virus (except, famously, AIDS) in creating a reasonably effective immune response. Sadly, the expected variants, which are already on their way, mean that no one is entirely protected forever.

The virus is very unlikely to spread, either as droplet or aerosol, in the very big, well-ventilated space that is the outdoors. Only people directly in front of an unmasked spreader are likely to encounter a significant amount of virus outside. So outdoor things are fine, as long as you don’t sit across a table from an unmasked, infectious person.

A restored wetland in the north suburbs. Photo by author

The amount of time spent in a room seems more problematic than distance from the infected person. If everyone enters at the same time, they should all be fine as long as they leave before the room’s CET is reached. So maybe we can have eight-hour conventions, or one-act shows, if the space is big enough. Of course, the more people there are in a space, the more likely it will be that more than one person in the room is infectious. So maybe not.

Sadly, the significant number is the amount of time spent in the room by the infectious person, not by you. If they have already been there long enough to fill the room with virus, you are in danger as soon as you enter. So bars and restaurants will not be safe unless each establishment is able to calculate their CET and is willing to use it to limit the time patrons can stay. In the meantime, come when no one else is there and don’t stay long.

Clearly, there are other factors involved in managing one’s own risk for catching COVID-19. If you’ve already had COVID-19, you are pretty safe, for the time being. But catching COVID-19 is of course not recommended as a long-term strategy. At this point, the best way to lower your risk is to get vaccinated. However, the chance that someone in a room with you is breathing out a variant of SARS-CoV-2 to which you are susceptible will decrease over time. But, even with vaccination, it will never reach zero.