COVID-19 may not meet the criteria for a seasonal infection – at least not yet – but that doesn’t mean there’s been no pattern to its peaks and troughs over the last few years. In a first detailed analysis, public health experts have revealed that the US has been seeing six-month waves of COVID that oscillate between north and south and, to a lesser extent, east and west. 

The study authors used freely available daily case rate data from the New York Times newspaper, covering the period between January 2020 – when COVID-19 first arrived in the USA – and August 2022, a total of 937 days. It amounted to 2.9 million data points, logging the date and location of a cumulative 95 million cases.

After analyzing the data at both state and county levels, some curious patterns began to emerge. 

“Our most striking finding was the alternating north–south intensity of COVID-19 case rates in the Eastern US,” the authors write in their paper. 

This wave that switches between northern and southern states, which the authors have termed the Eastern US COVID-19 Oscillation (EUCO), has not been recognized previously. As well as the nationwide peaks of COVID infections that have tended to be seen in the wintertime, the EUCO has driven “regional sub-epidemics”, according to the authors. 

They also identified a Northern US COVID-19 Oscillator (NUCO), which is less prominent than the EUCO but has been observed as a switching pattern between northeastern and north-central states. 

The findings help explain why the US has been seeing consistent peaks of infection during the summer months as well as the winter, which is unusual for most respiratory viruses. However, the authors write that the “drivers of these newly recognized oscillatory epidemic patterns remain to be elucidated.”

“The COVID-19 winter waves are consistent with that of other respiratory viruses, but the existence of a repeated additional surge during the summer was unexpected,” explained senior author Dr Donald S. Burke, dean emeritus of the University of Pittsburgh School of Public Health, in a statement. 

“These waves start near the southern US border in July and August, when the weather is hottest and the humidity is high – factors that usually tamp down the spread of respiratory viruses. We don’t have a good explanation for why COVID-19 rates should increase in both the warmest and coolest times of the year.”

The authors suggest that these oscillating waves might be indicative of a bigger pattern across the North American continent and recommend that their analysis be extended to investigate this. 

Getting a clearer understanding of COVID infection dynamics remains key for planning and forecasting. With new variants emerging regularly, understanding when and where peaks of infection may arise is important for the rollout of mitigations like testing and updated vaccines. 

More research will be needed to unpick exactly why we’re seeing these patterns, but one thing’s for sure – the more we learn about SARS-CoV-2, the more this virus continues to surprise us. 

The study is published in the journal Scientific Reports.