Flu Season Has Started But There’s Still Time to Benefit From Getting a Flu Vaccine

CDC Update

December 20, 2024 – Flu activity as tracked by CDC indicates that the 2024-2025 flu season has started. According to this week's FluView report, flu activity is increasing across the country, in all regions and across all age groups. Flu vaccination coverage in adults and pregnant women so far this year is similar to last season but is lagging among children. Overall, however, flu vaccination coverage is lower across most groups of people compared with before the COVID-19 pandemic. Flu activity is expected to continue for months so there's still time to benefit from a flu vaccine this season. Anyone who has not yet gotten a flu vaccine this season should get vaccinated now.

Flu Activity Update

Nationally, the percent of respiratory specimens testing positive for influenza virus, the percentage of emergency department visits for flu, the percentage of outpatient visits for respiratory illness, and flu-related hospitalizations increased this week compared to last week. This week, the percent of respiratory specimens testing positive for influenza virus (percent positivity) is at 9% nationally and increasing in all HHS Regions.

According to FluView, two jurisdictions (Louisiana and Oregon) reported very high influenza-like illness (ILI, fever plus cough or sore throat) activity, 13 jurisdictions reported high ILI, and 11 jurisdictions reported moderate ILI. Additionally, according to one of CDC's flu hospitalization surveillance systems, the National Healthcare Safety Network (NHSN), 8,912 people were hospitalized with flu this week. Two influenza-associated pediatric deaths were reported this week, bringing the total number of pediatric deaths for the season so far to four.

Surveillance data alone may underestimate the number of flu illnesses, hospitalizations, and deaths that occur because not everyone who gets sick with flu will seek medical care or be tested for influenza. To provide a more comprehensive estimate, CDC uses mathematical models to estimate of the total numbers of flu illnesses, flu-related hospitalizations, and flu-related deaths that occur during the season. So far this season, CDC estimates that there have been at least 1.9 million illnesses, 23,000 hospitalizations, and 970 deaths from flu.

Still Too Early to Assess Vaccine Performance

Flu vaccines vary in how well they work from season to season, but during seasons when vaccine viruses are similar to circulating viruses, vaccination has been shown to reduce the risk of flu illness by 40 percent and to reduce the risk of flu-related hospitalization by 60 percent. Flu vaccine effectiveness also can vary depending on what influenza viruses are spreading. Protection against influenza A(H1N1)pdm09 and influenza B viruses is usually higher than protection against influenza A(H3N2) viruses. According to this week's FluView report, cumulatively since September 29, 2024, influenza A viruses have accounted for 95.5% of influenza viruses reported to CDC by public health laboratories and influenza B viruses have accounted for 4.5% of viruses reported to CDC. Of the influenza A viruses that have been subtyped, 54.2% were influenza A(H3N2) viruses and 44.3% were influenza A(H1N1)pdm09 viruses.

CDC has genetically characterized 394 influenza viruses collected since September 29, 2024. Very few viruses have been antigenically characterized so far this season. Antigenic characterization refers to the analysis of the immune response generated by a virus. This is done to see how similar the virus is to the flu vaccine component. Antigenic characterization is done on a subset of viruses that have certain genetic changes. That means the viruses that are antigenically characterized are not proportional to the viruses spreading in the United States. To date, 34 influenza A(H3N2) viruses have been antigenically characterized, and 55.9% were antigenically similar to the A(H3N2) vaccine virus component. Fourteen A(H1N1)pdm09 viruses were antigenically characterized, and 100% were antigenically similar to the A(H1N1)pdm09 vaccine virus component. No influenza B/Victoria-lineage virus have been antigenically characterized yet. Additionally, it is important to highlight that antigenic characterization is based on the use of the ferret animal model (i.e., the use of ferret blood serum that contains antibodies, called "antisera"), and there are differences between the immune responses of ferrets and humans, which need to be considered when assessing and interpreting the antigenic properties of influenza viruses.

Flu vaccine effectiveness estimates based on real-world vaccine performance are expected in February 2025. A growing number of studies have shown that even when vaccinated people get sick with flu, their illness is less severe. It is also important to remember that flu vaccines protect against three different influenza viruses (two influenza A viruses and one influenza B virus). Additionally, multiple different influenza viruses usually circulate during any one season. For these reasons, CDC continues to recommend flu vaccination for everyone 6 months and older even if vaccine effectiveness against one or more viruses is reduced.

Flu Vaccination Coverage

As of November 30, 2024, 139.65 million doses of 2024-2025 flu vaccine have been distributed in the United States. Vaccine manufacturers projected that they will supply the United States with 148 million doses of influenza vaccines for the 2024–2025 season. These projections may change as the season progresses.

So far this season, flu vaccination coverage is lower among children compared to the same time last season. As of December 7, 2024, 40.8% of children received a flu vaccine, which is lower than last season at this same time point (44.2%). Flu vaccination coverage among children has decreased overall since prior to the COVID-19 pandemic. Flu vaccination coverage is about 10.4 percentage points lower than during the same time in 2019-2020 (51.2%).

As of December 7, 2024, the overall percentage of adults 18 years and older who have received a flu vaccine so far this season (40.8%) is similar to last season at this same time point (40.6%). Likewise, flu vaccination coverage is similar among people who received a flu vaccine during pregnancy (35%) compared with the same period in 2023 (35.1%). Non-Hispanic Asian people who received a flu vaccine during pregnancy had the highest coverage (49.7%), and non-Hispanic Black people who received a flu vaccine during pregnancy had the lowest coverage (29.2%). Flu vaccination coverage during pregnancy has decreased overall and among all racial and ethnic groups since prior to the COVID-19 pandemic. Flu vaccination coverage is about 16.8 percentage points lower than during the same time in the 2019-2020 season (51.8%).

There’s Still Time to Benefit from a Flu Vaccine This Season

While CDC recommends that people should ideally get a flu vaccine by the end of October, the agency continues to recommend flu vaccination as long as influenza viruses are circulating. Flu vaccination may also help protect people around you, including those who are at higher risk of serious flu illness, like babies and young children, adults 65 years and older, and people with certain chronic health conditions. Everyone 6 months and older should get a flu vaccine each year for the best protection against flu.

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