COVID-19 mystery: based on a true story

The mystery: ten friends gathered for a weekend pre-wedding party. After spending the weekend together, only five of them got COVID-19. Why did only half of the people get infected? It is highly likely they were infected with the Omicron variant—the dominant variant at this time—which is very contagious. Here’s how Scientific American described it: “The Omicron coronavirus variant was likely the fastest-spreading virus in human history. One person with the measles virus—a standout among infectious microbes—might infect 15 others within twelve days. But when Omicron suddenly arrived this past winter, it jumped from person to person so quickly that a single case could give rise to six cases after four days, 36 cases after eight days, and 216 cases after twelve days.” So asked another way, how did five party goers avoid getting Omicron? Read: Dr. Mary Fernando: Are we foolish enough to bail on the fourth dose? Let’s set the stage with what we know of protections against Omicro.n First, all those at the party had three doses of the COVID-19 vaccine—so there was no difference in vaccinations between those who got infected wth COVID-19 and those who didn’t. However, they were all about five months past their third dose of vaccine and vaccines wane, making them vulnerable to be infected at this point in time. None were eligible for their fourth doses because they were all in their twenties—too young for fourth dose eligibility. However, they took a very important precaution: understanding they were vulnerable to infection they all did rapid antigen tests on Friday morning before meeting on Friday afternoon. All of them tested negative. Did the rapid antigen tests miss an infection? How reliable are rapid antigen tests? Research suggests they aren’t entirely accurate, “Medical experts continue to warn that a negative result on a rapid test doesn’t necessarily mean you don’t have COVID-19.” New Swiss research, which is yet to be peer-reviewed, suggests some rapid tests have “significantly lower sensitivity” to Omicron than to the Delta variant. So, could this have been a problem with one or more people being infected with COVID-19 but not testing positive on the rapid antigen test? Unlikely, because they all spent a weekend together, not wearing masks and doing what young people do—hugging, laughing, occasionally bursting into song and sharing bedrooms. Is it possible that half the people avoided infection given all this? COVID-19 is airborne, so a person gets infected “when infectious particles that pass through the air are inhaled at short range,” a process otherwise known as “short-range aerosol or short-range airborne transmission”… transmission can (also) occur through “long-range airborne transmission” in poorly ventilated or crowded indoor settings “because aerosols can remain suspended in the air or travel farther than conversational distance.” If 10 young people share space with one or more infected persons, sharing the same air for an entire weekend, wouldn’t it be likely that more than half get infected? To answer this mystery, it’s helpful to fast forward to the end of the weekend. As all ten people were leaving, there were unmasked hugs, chats and laugher. Then five of the people piled into one car and five into the other. This is where the mystery becomes less baffling: all the people in one car got infected and none in the other car got infected. Again—the question is why? What was in one car but not the other? What was in one car but not present all weekend long that infected everyone in the car only? There are two important concepts to break down: the duration of safety with rapid antigen tests and the incubation period of Omicron. Rapid antigen tests, even if they accurately detect infections, are only useful for that moment in time. So, one can be accurately negative today and accurately positive tomorrow. How? Here we need to look at the incubation period of Omicron. An incubation period—for this mystery—is best seen as the time from the moment a person is infected to the time they are infectious. That time varies from three to five days. So lets’s backtrack to the week before the get together. Someone got infected with Omicron that previous week. They were asymptomatic when the weekend began and—very importantly—it was too early in the infection for the rapid antigen test to detect this early infection. As the weekend wore on, this person still remained infected but not infectious. However, on the two-hour long car ride home, this person went from infected to infectious and—with COVID-19 being airborne and no one wearing masks—everyone in that car became infected. We often say in medicine, timing is everything. It turns out, with rapid antigen tests and COVID-19, this still remains true. Dr. Mary Fernando is an Ottawa physician. This item originally appeared on SleuthSayers, a website for crime writers.

COVID-19 mystery: based on a true story

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