The flu season would have typically been well underway by mid-December in a normal year. But this year, there are few cases to speak of. Just a quick visit to the Centers for Disease Control and Prevention's (CDC) flu maps shows that this year activity is still mild everywhere in the United States; during many other years at this time, activity is often high. While some of that might be explained by fewer people coming to clinics for testing, that could not fully explain the dramatic drop in cases. It seems likely that efforts aimed at containing the more contagious SARS-CoV-2 pandemic virus have also crushed the influenza virus' ability to spread.
Last year, the flu season ended abruptly when lockdowns went into effect because of the pandemic. In the United States, the number of samples that were submitted for diagnosis did drop significantly - by 61 percent, but positive tests dropped far more - by 98 percent.
From April to July 2020, when the Southern Hemisphere went through winter, there was a shocking absence of a flu season. During that period, even though COVID-19 cases were rising, there were only 51 cases of flu detected in all of Australia, South Africa, and Chile even though 83,000 tests were administered.
“We know it’s less transmissible than coronavirus, so it makes sense,” Sonja Olsen, an epidemiologist at the National Center for Immunization and Respiratory Diseases told Nature. However, the decline was “greater than expected."
Virologist Richard Webby at St Jude’s hospital in Memphis, Tennessee added that “some South American countries haven’t done such a good job controlling COVID, but even there flu is low. I don’t think we can put it all down to mask-wearing and social distancing.” He suggested that the drop-off in international travel was a factor. Flu would normally maintain a low-level presence in the tropics year-round, and it would travel from one winter to another in different hemispheres over the course of the year. The mechanisms of this transmission aren't entirely clear but the movement of people plays an obvious part.
An increase in the number of flu vaccines that were administered may also be involved.
Rhinoviruses, which cause colds, have not followed the same trend as influenza. We know that they have persisted. “The one virus that’s not being halted is the rhinovirus,” Janet Englund, a pediatric infectious disease researcher at Seattle Children’s Hospital in Washington told Nature.
But we still don't know why that is. It seems that we will be untangling data that was generated over the course of 2020 on many subjects in the years to come.