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1/ Is the recent drop in US Covid cases due to an increased percentage of the population having immunity from previous infection or is it due to behavioral changes? In this thread I’ll show why I think behavioral changes are responsible. FYI: I have no credentials or expertise
2/ There have been numerous studies & articles noting that non-Covid seasonal respiratory infections have fallen off the table due to precautionary measures in place to protect against Covid. https://twitter.com/onisillos/status/1349781695629766657
3/ This has caused some to worry that the 2021-22 winter could be an unusually bad one for these diseases due to lowered immunity among the population from lack of exposure this year. See this study by @sang_woo_park @CJEMetcalf & others, for example. https://www.pnas.org/content/117/48/30547
4/ Regardless of what happens next winter with these seasonal respiratory infections, we've created an enormous natural experiment that should greatly increase our knowledge of disease cycles, pathogen diversity, and immunity, as @firefoxx66 points out. https://twitter.com/firefoxx66/status/1349788456218714116
5/ The first indication respiratory infections would be quashed by Covid precautions was that flu was virtually nonexistent in the southern hemisphere during their winter. That solid blue line isn’t the x-axis—it's influenza positivity for 2020. https://www.cdc.gov/mmwr/volumes/69/wr/mm6937a6.htm
6/ But influenza isn’t the only disease that’s seen its numbers dive; the same is true for virtually all seasonal respiratory diseases. Here is an area graph of the cumulative positivities of seven major categories of respiratory diseases in the US (via Biofire).
7/ Clearly the March lockdowns caused case numbers of these respiratory diseases to crater, & they’ve largely remained at extraordinarily low levels ever since, due to mask wearing, physical distancing, avoiding gatherings, & other anti-Covid measures.
8/ But there has been some variation in the prevalence of these respiratory diseases. They rose in June & July at the same time as the summer Covid surge in the South took place. And as Covid cases surged in November, so did the prevalence of these other respiratory diseases.
9/ This strongly suggests that the summer and late-fall Covid surges in the U.S. were due to human behavior (along with seasonality), not some mutation in SARS-CoV-2 or anything specific to Covid.
10/ And if you want to understand disease seasonality, this phenomenal review, "Seasonality of Respiratory Virus Infections" by @VirusesImmunity, Walter Hugentobler, & Miyu Moriyami is essential reading. My single favorite study I've read in the past year. https://www.annualreviews.org/doi/pdf/10.1146/annurev-virology-012420-022445
11/ Since Christmas, Covid cases have fallen. If this were due to the increasing percentage of the population already infected (or vaccinated) & therefore possessing immunity to SARS-CoV-2, we would expect the rise in other respiratory diseases to continue, or at least plateau.
12/ We do not see this. Instead, these respiratory illnesses have mirrored Covid, falling since Christmas. This strikes me as strong evidence that the recent drop in Covid cases is due to behavioral changes rather than population immunity. @yaneerbaryam @EricTopol
13/ This is consistent with this excellent analysis by @youyanggu, which found no correlation between the intensity of the fall/winter Covid surge and levels of previous infection in different U.S. states. https://twitter.com/youyanggu/status/1347266544946929665
14/ I’ve read that cell phone mobility data also indicate people have traveled less and become more cautious over recent weeks, but I don’t know how to find detailed mobility data. If someone else here does, please share.
