In 2022, Chile saw an influenza season with atypical timing and severity, indicating similar cases may occur in regions such as the United States.

Influenza may be more severe and strike with different timing than usual during the 2022 to 2023 influenza season, based on data from the Southern Hemisphere.

Throughout the COVID-19 pandemic, influenza infection rates have been drastically reduced. This decline has been attributed to measures taken to reduce the spread of COVID-19, such as social distancing and implementation of face masks. As these measures are relaxed, lack of population exposure to influenza could cause a severe influenza epidemic.

The start of the influenza season is measured as the epidemiologic week where incidence rates have remained above the epidemic threshold for 3 or more weeks. The mean of positive cases weekly is used to define the influenza season.

Data from Chile’s most recent influenza season was compared to prior influenza seasons to estimate the effects of influenza in other regions. Chile’s latest influenza season began in January 2022, earlier than prepandemic seasons.

Influenza incidence was gathered from hospital discharge data, which was estimated by Chile’s Ministry of Health Department of Statistics and Health Information. Surveillance data was also gathered from the National Influenza Centre.

Pneumonia and influenza (P&I) hospitalizations were 5.1 per 100,000 persons in the recent season, which was higher than pandemic seasons but lower than the 2017 to 2019 influenza seasons.

Following the initial influenza wave, Chile’s government vaccinated 88% of the targeted population for influenza. This targeted population made up 41% of Chile’s total population. When adjusting for vaccine effectiveness, associated hospitalization was 49%.

In some patients, influenza was tested using the Center for Disease Control of Prevention’s reverse transcription–polymerase chain reaction. Incidence of influenza hospitalization was estimated by applying the percentage of patients with severe acute respiratory infections tested positively for influenza to untested patients diagnosed with P&I. 

These rates were calculated each month for each age group. Cases were only estimated when P&I was the primary factor for hospitalization.

From January to August 2022, 17,752 individuals in Chile were hospitalized for P&I. Of these patients, 27.7% were aged under 5 years and 5.2% were aged 5 to 18 years. Influenza-attributed P&I was 11.2 and 2.2 for these age groups respectively.

Investigators believed that Northern Hemisphere regions could see similar influenza data and encouraged prevention measures. These include influenza vaccination and avoiding close contact with individuals afflicted with the disease.

Celeste Krewson, Assistant Editor