The virus that causes COVID-19, SARS-CoV-2, has transitioned from a constant pandemic threat to a predictable, cyclical respiratory illness in many regions. This shift means the virus now exhibits periods of higher and lower activity, similar to influenza and other endemic coronaviruses. Recognizing the factors that drive these peaks allows the public to prepare for times when transmission risk is higher, moving toward routine seasonal health management.
Environmental and Behavioral Drivers of Seasonal Peaks
The seasonality of COVID-19 transmission is partly explained by the physical properties of the virus and the surrounding environment. Cold and dry conditions moderately increase the transmissibility of SARS-CoV-2. Lower temperature and lower specific humidity are associated with an increased reproduction number because the virus is more stable and infectious when the air is less humid, allowing aerosolized particles to remain suspended longer.
Changes in air quality also play a role, as cold air can impact the respiratory tract’s immune defenses. Exposure to colder air may reduce the mucosal lining’s ability to clear viral particles, making the host more susceptible to infection. These environmental factors combine with human behavior to create peak transmission periods.
Human behavior provides a significant driver for the seasonal peaks, particularly during cooler months. As temperatures drop, people spend more time indoors, often in crowded or poorly ventilated spaces. This increased indoor mixing, especially in shared spaces like schools and workplaces, facilitates the efficient spread of an airborne virus. The combination of increased viral stability in dry winter air and higher-density indoor contact accelerates the rate of new infections.
Timing and Co-circulation with Other Respiratory Viruses
COVID-19 activity typically follows a primary annual window, with hospitalizations generally peaking in late December or early January in the Northern Hemisphere. However, the virus has shown a less consistent pattern than influenza, sometimes including smaller surges during the summer months linked to increased travel or the emergence of new variants. This irregular behavior underscores that while seasonality exists, the exact timing varies year-to-year based on viral evolution and population immunity levels.
Peak COVID-19 activity often coincides with the circulation of other common respiratory pathogens, a phenomenon referred to as co-circulation. The most notable viruses involved are Influenza and Respiratory Syncytial Virus (RSV). RSV epidemics frequently peak before influenza, often by about three weeks, straining healthcare systems earlier in the season.
The overlap of these three viruses—COVID-19, Influenza, and RSV—creates a challenging scenario for public health officials and clinicians. Because all three cause similar respiratory symptoms, diagnosis and treatment selection become more complex during the late fall and winter. This period requires strategic testing to differentiate between the pathogens, allowing for appropriate use of available antiviral treatments.
Adapting Public Health Measures for Peak Activity
Preparing for the seasonal increase in COVID-19 activity involves a multi-layered approach centered on vaccination and improving indoor environments. Vaccination timing is important to maximize protection during the highest-risk period of late fall and winter. For most individuals in the Northern Hemisphere, receiving the updated annual COVID-19 vaccine in September or early October is optimal.
This early autumn timing ensures peak antibody levels are established just before the virus begins its typical surge in November and December. If a person has recently had a COVID-19 infection, delaying the vaccine by about three months is recommended to allow natural immunity to mature. The updated vaccine targets the variants most likely to be circulating in the coming season.
Non-pharmaceutical interventions, particularly those focused on air quality, are effective during peak periods. Improving indoor air quality involves ventilation and filtration to reduce the concentration of airborne viral particles. This can be achieved by increasing the amount of outdoor air brought indoors, such as by opening windows or maximizing the intake of an HVAC system.
Filtration is another measure, which can be accomplished by upgrading HVAC filters to a Minimum Efficiency Reporting Value (MERV) 13 rating or higher, or by using portable air cleaners with High-Efficiency Particulate Air (HEPA) filters. Using at-home tests when symptoms first appear allows for prompt diagnosis and access to antiviral treatments, which are most effective when started early.