The Absence of Influenza

Between March 13^th and March 24^th, over 50 million American children were sent home from school in an unprecedented effort to arrest the spread of SARS-CoV-2. School closure is one element of nonpharmaceutical interventions for pandemic response. Nearly immediately, my research team and I noticed the sudden departure of influenza. Influenza is typically on its way out by late March, but this year’s decline was precipitous.

There have now emerged reports of surprisingly low levels of influenza across Southern Hemisphere countries that experience similar seasonality to the US, including Australia, Argentina, Chile, New Zealand, and South Africa.¹ For example, Chile is currently reporting a case burden that is only 5% of that at the same point in last year’s influenza outbreak.

The idea of influenza’s hemispheric oscillation emerged from an old and seminal paper from 1981.² The influenza season in the Southern Hemisphere is offset from ours by 6 months and typically runs from April to September. Accordingly, I have learned to watch Australia’s surveillance to gauge our upcoming influenza season, and the current trends are reassuring.

The implementation of nonpharmaceutical interventions—including school closure, physical distancing, use of face masks, and hand hygiene—have created unfavorable conditions for influenza transmission. If these continue here though our fall and winter, we may catch a break from influenza. This, however, does not exempt us from being aggressive with the use of influenza vaccines as soon as they become available. We go into late summer and fall with uncertainty about SARS-CoV-2, and influenza vaccine may help reduce symptomatic respiratory infections and reduce diagnostic confusion.

One final caution… Australia may also provide a foreshadowing of COVID-19 things to come. Following a largely successful containment of SARS-CoV-2 in April (autumn), COVID-19 cases came back with a vengeance in July (winter), showing that this virus may have some features of a seasonal coronavirus.