According to the Centers for Disease Control and Prevention (CDC) and other virus trackers, aside from the coronavirus, respiratory and gastrointestinal viruses are circulating at or near the lowest levels ever previously recorded. Cases of the respiratory bacteria that cause pertussis are at a record low, as are influenza A, influenza B, parainfluenza, norovirus, respiratory syncytial virus, and human metapneumovirus.
During the third week of December 2019, the CDC reported that 16.2% of 29,578 samples tested were positive for influenza A, compared with just 0.3% during the same week in 2020.
BioFire, a company that provides diagnostic tests for viral and bacterial infections to more than 2,500 U.S. health care providers, maintains a chart illustrating positivity rates for 13 viral and bacterial respiratory illnesses over the last two years. Their records show that in January 2020, nearly 60% of samples from patients with flu-like symptoms tested positive for one of these pathogens, compared with 18% in January 2021.
Epidemiologists fear a potential rebound of these viruses later, after COVID-19 cases decline and social distancing loosens as people resume their normal routines. “The best analogy is to a forest fire,” said Bryan Grenfell, DPhil, an epidemiologist and population biologist at Princeton University. “For the fire to spread, it needs to have unburned wood. For epidemics to spread, they require people who haven’t previously been infected. So if people don’t get infected this year by these viruses, they likely will at some point later on.”
However, social distancing isn’t the only factor potentially suppressing these illnesses. Walgreens Chief Medical Officer Kevin Ban, MD, told The Washington Post that the pharmacy chain has seen “unprecedented demand” for flu shots this season.
Additionally, viral interference – the immune system’s interferon response – could be blocking the spread of other viruses. “The interferon response is one of the body’s best defenses against respiratory viruses,” said Ellen Foxman, MD, PhD, Assistant Professor of Laboratory Medicine and Immunobiology at Yale University. “As soon as it gets turned on by one virus, any other virus that comes along and tries to grow in the respiratory tract can’t. [Given that] we’ve never had a natural experiment like this before, what the consequence will be for the short and long term remains to be seen.”