COVID-19 May Follow A Different Seasonal Pattern Than Influenza, RSV

Researchers have observed an uptick in SARS-CoV-2 levels in wastewater, alongside slight increases in other respiratory viruses, as temperatures rise, according to the most recent report from BioBot, a wastewater intelligence platform.¹

Although overall respiratory virus activity remains low nationwide, the increase aligns with previous research suggesting that COVID-19 transmission can rise during the spring and summer months, particularly in the southern US. These observations parallel recent trends reported by the CDC, which indicate that COVID-19 activity is growing or likely to grow in Texas, Louisiana, and Mississippi.²

Current SARS-CoV-2 wastewater levels remain low nationally, and the CDC does not predict a summer surge. However, officials note that future activity could increase if a substantially immune-evasive variant were to emerge. Such scenarios reinforce the importance of continued surveillance rather than serving as forecasts of an imminent wave.³

Research Suggests COVID-19 Experiences Spring and Summer Waves in Addition to Winter Peaks

Unlike influenza and respiratory syncytial virus (RSV), which generally follow predictable fall and winter seasonal patterns, SARS-CoV-2 has demonstrated a more variable transmission pattern since the beginning of the pandemic. Researchers attribute these differences to a combination of waning population immunity, the emergence of new variants, regional climate differences, and human behavior rather than temperature alone.

One study examining COVID-19 incidence across the US identified 3 recurring annual increases in cases: a primary peak during the early- to mid-winter months followed by 2 smaller waves occurring in the spring and again during mid- to late summer.⁴ During both the 2020-2021 and 2021-2022 respiratory seasons, spring activity peaked around mid- to late April, while the second increase occurred between late July and August.⁴

In April 2026, reports of the BA.3.2 SARS-CoV-2 lineage, commonly referred to as the Cicada variant, also increased, accompanied by greater detection in wastewater surveillance. Although evidence regarding the variant's transmissibility remains limited, experts note that BA.3.2 contains more than 70 mutations, several of which may contribute to immune evasion rather than inherently increasing transmissibility.⁵ Continued monitoring will be necessary to determine whether the variant alters seasonal transmission patterns.

Regional Climate and Human Behavior May Help Explain Summer COVID-19 Transmission

Additional research has shown that COVID-19 transmission frequently oscillates between northern and southern regions of the US throughout the year. During the summer months, increases have historically been observed in southern states, including Texas, Louisiana, and Mississippi, before shifting northward during the winter months.⁶

Rather than warm weather itself driving transmission, researchers suggest that seasonal behavioral and environmental factors likely contribute to these regional differences. High temperatures and humidity often encourage people to spend more time indoors in air-conditioned environments where prolonged close contact may facilitate viral spread. Summer travel, gatherings, and declining immunity from prior vaccination or infection may also contribute to seasonal increases.⁶

These observations differ from influenza and RSV, which remain predominantly winter respiratory viruses despite occasional fluctuations in timing. Influenza transmission is favored by colder, drier conditions, while RSV typically peaks from late fall through winter. In contrast, SARS-CoV-2 continues to exhibit a more flexible seasonal pattern that appears to be shaped by viral evolution and changes in population immunity in addition to environmental factors.

Researchers caution that these recurring patterns should not be interpreted as fixed predictions. The timing and magnitude of future COVID-19 waves will continue to depend on the emergence of new variants, population immunity, vaccination uptake, and local environmental conditions. Nevertheless, the findings underscore the value of wastewater surveillance as an early warning tool that can detect increasing viral circulation before clinical cases begin to rise, allowing health systems and public health officials additional time to prepare.

References

1. Donnelly M. COVID-19, influenza, and RSV wastewater monitoring in the US: week of June 13, 2026. BioBot. June 22, 2026. Accessed June 29, 2026. https://biobot.io/risk-reports/covid-19-influenza-and-rsv-wastewater-monitoring-in-the-u-s-week-of-june-13-2026

2. CFA: Modeling and forecasting: current epidemic trends (based on R_t). CDC. June 26, 2026. Accessed June 29, 2026. https://www.cdc.gov/cfa-modeling-and-forecasting/rt-estimates/index.html

3. Respiratory illnesses: respiratory illnesses data channel. CDC. June 26, 2026. Accessed June 29, 2026. https://www.cdc.gov/respiratory-viruses/data/index.html

4. Shamsa EH, Shamsa A, Zhang K. Seasonality of COVID-19 incidence in the United States. Front Public Health. 2023;11. doi: 10.3389/fpubh.2023.1298593

5. McCrear S. Cicada COVID-19 variant FAQs: symptoms, risk, and prevention. AJMC^®. June 4, 2026. Accessed June 29, 2026. https://www.ajmc.com/view/cicada-covid-19-variant-faqs-symptoms-risk-and-prevention

6. Jala H, Lee K, Burke DS. Oscillating spatiotemporal patterns in COVID-19 in the United States. Sci Rep. 2024;14(1):21562. doi: 10.1038/s41598-024-72517-6