Influenza Activity in the United States during the 2022–23 Season and Composition of the 2023–24 Influenza Vaccine

CDC does not know the exact number of people who have been sick and affected by influenza because influenza is not a reportable disease in most areas of the United States. However, CDC uses a mathematical model to estimate the number of influenza illnesses, medical visits, hospitalizations, and deaths that occur each season (5). Preliminary in-season estimates of influenza burden in the United States for the 2022-23 season are that influenza virus infection resulted in 31 million symptomatic illnesses, 14 million medical visits, 360,000 hospitalizations, and 21,000 deaths. Preliminary end of season burden estimates for the 2022-23 season will be available in October 2023.

Influenza activity in the Southern Hemisphere typically occurs between April and September and can last into October or November. In the Northern Hemisphere, the influenza season can begin as early as October and can last as late as April or May. According to data collected and submitted by countries to the World Health Organization’s FluNet system (6), influenza activity in the Southern Hemisphere is ongoing as of August 2023, with influenza A viruses being reported most often and influenza A(H1N1)pdm09 being the predominant subtype.

Some South American countries experienced early or high levels of influenza activity compared to their historic trends. For example, activity in Chile began earlier than historic trends, while activity in Paraguay ended earlier than usual, and activity in Bolivia was both higher and earlier compared to historic data. South Africa, one of the few Southern Hemisphere countries with predominantly influenza A(H3N2) virus activity, experienced a high percentage of respiratory specimens testing positive for influenza but a moderate level of influenza-related hospitalizations compared to historic trends (7). Activity in South Africa declined earlier than what has been seen historically. Similar to South Africa, influenza activity in Australia decreased earlier during the 2023 season as compared to historic trends.

Of note, Mexico, located in the Northern Hemisphere, traditionally experiences peak influenza circulation during the October‒March but had atypically high influenza activity during the Southern Hemisphere season and saw primarily influenza B/Victoria virus circulation. In the tropical countries of Bangladesh and Thailand, the 2023 influenza season began later than previous seasons and activity is still above epidemic thresholds.

The 2022-23 influenza season in the United States is considered  moderately severe  with an estimate of at least 31 million symptomatic illnesses, 14 million medical visits, 360,000 hospitalizations, and 21,000 deaths caused by influenza virus infection. The predominant influenza virus throughout the 2022-23 influenza season was influenza A(H3N2); however, as activity declined, A(H1N1)pdm09 and B/Victoria viruses began circulating at higher proportions than A(H3N2) viruses. Most influenza viruses tested were in the same genetic subclade as, and antigenically similar to, the vaccine reference viruses included in the season’s influenza vaccine. All the influenza viruses collected and tested for antiviral resistance by CDC since October 2, 2022, were susceptible to zanamivir, and peramivir and the majority (>99%) were susceptible to oseltamivir and baloxivir.

This season marked a return to influenza activity at levels more similar to those seen prior to the COVID-19 pandemic. The peak percentage of specimens testing positive for influenza (26.3%) as reported to CDC from clinical laboratories nationally was similar to the average peak percentage positive (26.5%) during the five influenza seasons (2015-16 – 2019-20) immediately preceding the COVID-19 pandemic. Similarly, the cumulative rate of influenza-associated hospitalizations as reported in FluSurv-NET this season was similar to hospitalization rates for 4 pre-COVID seasons (2014-15, 2016-17, 2018-19, and 2019-20 seasons) and higher than all but one remaining season (2017-18 season) since the 2010-11 season.

Mortality attributed to influenza also returned to levels more similar to what was seen before the COVID-19 pandemic. As of September 14, 2023, the number of death certificates with influenza listed as an underlying or contributing cause of death during the 2022-23 season (9,697) was above the average number of influenza coded deaths (8,530) during the five seasons preceding the COVID-19 pandemic (2015-16 through 2019-20) and 3 to 10 times higher than the number of influenza coded deaths during the first two years of the COVID-19 pandemic. The number of pediatric deaths reported this season (174) was above the average reported number of deaths each season (147) for the five seasons preceding the COVID-19 pandemic.

Although influenza activity in the United States returned to levels similar to pre-COVID-19 influenza seasons, timing of activity occurred earlier than usual. During the 2022-23 season, activity peaked nationally between late November and early December (depending on the indicator) which is at least three weeks before the earliest recorded seasonal peak going back to the 1997-1998 season and two months before the most common month for peak influenza activity (February). Influenza activity returned to interseasonal levels in February 2023, and no significant second wave of activity was reported.

While not always a predictor of the subsequent Northern Hemisphere influenza season, Southern Hemisphere activity in the summer is often considered when planning for what could be expected. However, due to continued varied influenza activity among Southern Hemisphere countries, different influenza viruses predominating in different parts of the world, and the possibility of variations in population immunity between the Northern and Southern Hemispheres, Southern Hemisphere activity cannot be used to definitively predict  the timing or intensity of influenza activity in the United States this fall and winter.

Though influenza activity has remained low in the summer, maintaining vigilance for influenza virus infections in the United States year-round is important. Sporadic seasonal influenza virus infections and novel influenza A infections associated with exposure to swine during animal exhibitions often are reported during the summer months (8). While the number of outbreaks of highly pathogenic avian influenza A(H5N1) virus among birds has been low in this summer it is important that providers and persons with exposure to sick or infected birds remain attentive to any new symptoms that could be consistent with influenza virus infection (4). Patients with suspected novel influenza A virus infection should isolate at home away from household members and not go to work or school until they are proven not to be infected or have recovered from their illness. Specimens from patients with suspected novel influenza A virus infection should be collected and referred to state public health departments for testing and treatment with influenza antiviral medications should be initiated immediately.

While we do not know exactly what the coming influenza season will look like, influenza results in a significant public health burden in the United States every winter. For persons aged ≥6 months, receiving a seasonal influenza vaccine each year remains the best way to protect against seasonal influenza and its potentially severe consequences. There are also everyday preventive actions, including avoiding close contact with people who are sick, limiting contact with others if you are sick, and covering your coughs and sneezes, that can help reduce the spread of influenza. Influenza antiviral drugs are another way to minimize the impact of influenza this coming season. CDC recommends initiating influenza antiviral drug treatment as soon as possible for patients with confirmed or suspected influenza virus infection who have severe, complicated, or progressive illness; who require hospitalization; or who are at increased risk for influenza-associated complications (9). Four influenza antiviral drugs approved by the Food and Drug Administration are currently recommended for use in the United States.