Seasonal trends: why COVID-19 rises in winter and summer

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Why COVID-19 cases can increase in the winter

Scientists have observed that COVID-19 tends to rise during colder months, similar to the flu and other respiratory viruses. But what causes this seasonal spike?

Travel and events

Winter holidays, family gatherings, and seasonal travel bring people together from different households.

Cold, dry air

Cooler temperatures and lower humidity levels create ideal conditions for respiratory viruses. Dry air allows viral particles to remain airborne longer, increasing the chance of being inhaled.

Indoor crowding

When temperatures drop, people naturally spend more time indoors. This leads to closer contact in enclosed spaces with limited airflow, which are ideal conditions for viral transmission. Therefore, holiday gatherings and travel during winter months may increase the risk of exposure.

Ventilation and air quality

Indoor environments often lack proper ventilation, especially in older buildings or homes sealed tightly against the cold. Poor air circulation allows viral particles to accumulate, increasing the risk of infection.

Changes to the immune system

Studies suggest that our immune systems may be less effective in colder weather. Reduced sunlight exposure can lower vitamin D levels, which play a role in immune function.

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Why COVID-19 cases can rise in the summer

Summer increases in COVID-19 might seem surprising, but they do occur. What causes increases in infections during the summer months?

Travel and events

Summer vacations, weddings, barbeques, and festivals bring large groups together, increasing the opportunity for person-to-person transmission.

Indoor spaces

In hot climates, people often retreat indoors to air conditioning. Being in enclosed places with limited airflow can increase the risk of transmission.

Relaxed precautions

General precautions often decline in summer, as many people incorrectly assume COVID-19 cases drop during warmer months, or initially dismiss symptoms as being related to seasonal allergies.

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How new variants can cause COVID-19 cases to rise

COVID-19 viruses evolve over time. They can change (mutate), creating new variants that can evade the immune response provided by prior infection or vaccination. These variants can drive new waves of cases, regardless of the season.

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Understanding seasonal rises in COVID-19 infections

To summarize, COVID-19 cases can rise at any time, but in winter and summer we often see increases in infections. Understanding why increases occur can help you be more aware that "seasonal symptoms" might be more than just allergies or the common cold.

Colder weather, dry air, indoor gatherings, and weakened immune responses all contribute to higher viral spreading during certain times of the year. Summer rises can result from travel, crowded events, and gathering indoors in air-conditioned spaces.

By recognizing these seasonal factors, you can take proactive health measures and be prepared to act quickly if needed. 3 out of 4 adults in the U.S. are at risk of severe illness, so take the COVID-19 Risk Assessment to find out your likelihood of severe illness, and talk to a healthcare provider about prescription options if you are feeling sick or test positive.