It doesn’t follow normal seasonal patterns, like other respiratory viruses – waves of infection can happen at any time of year.
Covid-19 doesn’t adhere to the usual seasonal patterns seen in other respiratory viruses; waves of infection can occur at any time of the year. Each July over the past four years, epidemiologists at the US Centers for Disease Control and Prevention (CDC) have observed a sudden rise in Covid-19 cases and hospitalizations, an annual event termed “the summertime surge”
This summer, Covid-19 rates are notably high in Arizona, California, Hawaii, and Nevada, with positive test rates reaching 15.6% in the week ending July 6th, a 1% increase from the previous week. The CDC’s investigations also indicate rising viral levels in wastewater.
Similar trends are observed in the UK, where the Health Security Agency reports that positive Covid-19 tests increased from 4% at the end of March to 14% by the end of June. The recent rise is linked to the FLiRT subvariants, the latest evolution of the Omicron strain of Covid-19, which emerged in late 2021. These subvariants have adapted by mutating their spike protein, which helps the virus evade the immune system while still binding effectively to the ACE2 receptor on human cells, facilitating entry into the body.
Shan-Lu Liu, director of the Viruses and Emerging Pathogens Program at The Ohio State University, notes that these new subvariants strike a balance between immune evasion and cell binding, driving many new cases. He emphasizes to the BBC that the elderly and immunosuppressed are particularly vulnerable due to their weakened immune responses.
Experts recommend booster shots, including the XBB.1.5 monovalent vaccine, designed to target the Omicron XBB 1.5 subvariant.
Studying these emerging variants is essential for updating Covid-19 vaccines, enabling regulators like the CDC and the World Health Organization to make specific recommendations for new vaccine targets. However, predicting when significant new strains will appear remains challenging. Unlike common respiratory infections like influenza or Respiratory Syncytial Virus (RSV), which follow seasonal patterns, Covid-19 has not settled into a predictable cycle.
Factors contributing to the summer Covid-19 wave include gatherings at festivals and concerts and the use of air conditioning, which can promote viral spread. For example, in the UK, increased gatherings during the Euro 2024 football tournament likely contributed to the rise in cases.
The year-round effect of Covid-19, unlike other respiratory infections, is attributed to lower population immunity. Older viruses like influenza have been around longer, resulting in higher immunity levels that require ideal conditions for infection during colder months. In contrast, Covid-19 is relatively new, with lower sterilizing immunity and low vaccination rates, making population immunity dependent on recent infection waves.
Experts believe Covid-19 may eventually adopt a more seasonal pattern, but this transition could take years or even decades. Some evidence suggests a trend towards seasonality, with milder summer surges compared to winter peaks. However, achieving a stable seasonal pattern and integrating it with existing seasonal viruses like RSV and flu remains a long-term challenge. Public health messaging needs to emphasize year-round vaccination and antiviral access, especially for the most vulnerable populations.
Overall, while Covid-19’s future seasonality remains uncertain, continuous monitoring and adaptation of public health strategies are crucial.
