I was super sick in Nov/Dec/Jan (or in most parts of the world, February). Did I have COVID-19 already?

Biology/Immunity Infection and Spread

A: Nope… you probably had the flu. What makes us think it wasn’t COVID-19?

We had a bad influenza year in the United States. This year’s flu vaccine did not cover one of the main flu strains that circulated this year very well–a type of influenza type B. So a lot of people who got the vaccine did get pretty sick with flu last winter.

Influenza is a virus totally unrelated to coronaviruses–not even the same viral family. However, flu can cause very similar symptoms–fever, chills, prolonged fatigue, and a cough that can last a while. Influenza kills many thousands of people worldwide each year, and it’s probably the most serious infectious disease we still face routinely in the United States and Europe. In the 2019-2020 winter flu season in the United States, 45 million people are estimated to have had influenza, and between 24,000 and 64,000 died of influenza.

Interestingly enough (well, I think it’s interesting, but then again I am *famously* nerdy), the influenza season was curtailed very abruptly by the social distancing measures put in place in many localities as COVID-19 broke across the United States.

So if there were no COVID19 tests last winter, and you never got tested for flu either, why would we assume that those two miserable weeks you spent in bed were not coronavirus?

Scientists can do some fancy genetic magic to identify how long ago the virus was introduced into circulation in a particular area (and also where it was introduced from, geographically speaking) to date an epidemic retrospectively. They use a method called genomic sequencing to find tiny mutations in the viral genetic code, and use them to date a particular sample of the virus.

Genetic sequencing studies indicate that the very first cases in the world occurred in Wuhan, China, and that the virus from which all our current cases descended–the one that sparked it all–mutated to a form that can infect humans no earlier than October, 2019.

The first human case anyone identified as being something unusual was a patient with “pneumonia of unknown origin,” on December 8th in a hospital in China.

By the time Chinese authorities had identified dozens of such cases and reported a novel infectious disease outbreak to the World Health Organization on December 31st, it had already spread widely in that region.

The very first cases were introduced to the US in the Seattle, Washington area in mid-January. That’s the earliest anyone in the States had it. This timeline for US introduction has three pieces of evidence that support it. 1. The travel and exposure patterns of people who fell ill (and were tested, in some cases). 2. Genomic sequencing studies that date the virus. And 3. A rather clever study where scientists repurposed old influenza test samples to look for long-gone COVID19 infections.

Genomic sequencing indicates that between mid-January to late February, those first few cases in the Seattle area (or maybe genetically identical cases imported directly from Wuhan) spread across the United States and around the globe. It hopped around a fair amount from one city to another–probably hitching a ride on air travelers. By late February to the first week of March, we were seeing community spread (that is, people getting sick who had not traveled elsewhere but caught it locally) in the United States. So if you had flu-like symptoms in the United States in November, December, or January… in all likelihood, you were one of the millions of Americans who had the flu this year.

Similar studies indicate that the virus spread from China to South Korea in early January and Italy and Iran in late January.

NY Times Article 1

NY Times Article 2

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