Update: Influenza Activity in the United States During the 2018-19 Season and Composition of the 2019-20 Influenza Vaccine.

Influenza activity* in the United States during the 2018-19 season (September 30, 2018-May 18, 2019) was of moderate severity (1). Nationally, influenza-like illness (ILI)† activity began increasing in November, peaked during mid-February, and returned to below baseline in mid-April; the season lasted 21 weeks,§ making it the longest season in 10 years. Illness attributed to influenza A viruses predominated, with very little influenza B activity. Two waves of influenza A were notable during this extended season: influenza A(H1N1)pdm09 viruses from October 2018 to mid-February 2019 and influenza A(H3N2) viruses from February through May 2019. Compared with the 2017-18 influenza season, rates of hospitalization this season were lower for adults, but were similar for children. Although influenza activity is currently below surveillance baselines, testing for seasonal influenza viruses and monitoring for novel influenza A virus infections should continue year-round. Receiving a seasonal influenza vaccine each year remains the best way to protect against seasonal influenza and its potentially severe consequences.

Update: Influenza Activity in the United States During the 2017-18 Season and Composition of the 2018-19 Influenza Vaccine.

The United States 2017-18 influenza season (October 1, 2017-May 19, 2018) was a high severity season with high levels of outpatient clinic and emergency department visits for influenza-like illness (ILI), high influenza-related hospitalization rates, and elevated and geographically widespread influenza activity across the country for an extended period. Nationally, ILI activity began increasing in November, reaching an extended period of high activity during January-February, and remaining elevated through March. Influenza A(H3N2) viruses predominated through February and were predominant overall for the season; influenza B viruses predominated from March onward. This report summarizes U.S. influenza activity* during October 1, 2017-May 19, 2018.†.

Update: Influenza Activity - United States, October 1, 2017-February 3, 2018.

Influenza activity in the United States began to increase in early November 2017 and rose sharply from December through February 3, 2018; elevated influenza activity is expected to continue for several more weeks. Influenza A viruses have been most commonly identified, with influenza A(H3N2) viruses predominating, but influenza A(H1N1)pdm09 and influenza B viruses were also reported. This report summarizes U.S. influenza activity* during October 1, 2017-February 3, 2018,† and updates the previous summary (1).

Update: Influenza Activity - United States, October 1-November 25, 2017.

Influenza activity in the United States was low during October 2017, but has been increasing since the beginning of November. Influenza A viruses have been most commonly identified, with influenza A(H3N2) viruses predominating. Several influenza activity indicators were higher than is typically seen for this time of year. The majority of influenza viruses characterized during this period were genetically or antigenically similar to the 2017-18 Northern Hemisphere cell-grown vaccine reference viruses. These data indicate that currently circulating viruses have not undergone significant antigenic drift; however, circulating A(H3N2) viruses are antigenically less similar to egg-grown A(H3N2) viruses used for producing the majority of influenza vaccines in the United States. It is difficult to predict which influenza viruses will predominate in the 2017-18 influenza season; however, in recent past seasons in which A(H3N2) viruses predominated, hospitalizations and deaths were more common, and the effectiveness of the vaccine was lower. Annual influenza vaccination is recommended for all persons aged ≥6 months who do not have contraindications. Multiple influenza vaccines are approved and recommended for use during the 2017-18 season, and vaccination should continue to be offered as long as influenza viruses are circulating and unexpired vaccine is available. This report summarizes U.S. influenza activity* during October 1-November 25, 2017 (surveillance weeks 40-47).†.

Update: Influenza Activity in the United States During the 2016-17 Season and Composition of the 2017-18 Influenza Vaccine.

During the 2016-17 influenza season (October 2, 2016-May 20, 2017) in the United States, influenza activity* was moderate. Activity remained low through November, increased during December, and peaked in February nationally, although there were regional differences in the timing of influenza activity. Influenza A(H3N2) viruses predominated through mid-March and were predominant overall for the season, but influenza B viruses were most commonly reported from late March through May. This report summarizes influenza activity in the United States during October 2, 2016-May 20, 2017† and updates the previous summary (1).

Update: Influenza Activity - United States, October 2, 2016-February 4, 2017.

This report summarizes U.S. influenza activity* during October 2, 2016-February 4, 2017,† and updates the previous summary (1). Influenza activity in the United States began to increase in mid-December, remained elevated through February 4, 2017, and is expected to continue for several more weeks. To date, influenza A (H3N2) viruses have predominated overall, but influenza A (H1N1)pdm09 and influenza B viruses have also been identified.

Update: Influenza Activity - United States, October 2-December 17, 2016.

This report summarizes U.S. influenza activity* during October 2-December 17, 2016.† Influenza activity in the United States remained low in October and has been slowly increasing since November. Influenza A viruses were identified most frequently, with influenza A (H3N2) viruses predominating. Most influenza viruses characterized during this period were genetically or antigenically similar to the reference viruses representing vaccine components recommended for production in the 2016-17 Northern Hemisphere influenza vaccines.

Update: Influenza Activity - United States and Worldwide, May 22-September 10, 2016.

During May 22-September 10, 2016,* the United States experienced typical low levels of seasonal influenza activity overall; beginning in late August, clinical laboratories reported a slight increase in influenza positive test results and CDC received reports of a small number of localized influenza outbreaks caused by influenza A (H3N2) viruses. Influenza A (H1N1)pdm09, influenza A (H3N2), and influenza B viruses were detected during May-September in the United States and worldwide. The majority of the influenza viruses collected from the United States and other countries during that time have been characterized antigenically or genetically or both as being similar to the reference viruses representing vaccine components recommended for the 2016-17 Northern Hemisphere vaccine. During May 22-September 10, 2016, 20 influenza variant virus† infections were reported; two were influenza A (H1N2) variant (H1N2v) viruses (Minnesota and Wisconsin) and 18 were influenza A (H3N2) variant (H3N2v) viruses (12 from Michigan and six from Ohio).

Influenza Activity - United States, 2015-16 Season and Composition of the 2016-17 Influenza Vaccine.

During the 2015-16 influenza season (October 4, 2015-May 21, 2016) in the United States, influenza activity* was lower and peaked later compared with the previous three seasons (2012-13, 2013-14, and 2014-15). Activity remained low from October 2015 until late December 2015 and peaked in mid-March 2016. During the most recent 18 influenza seasons (including this season), only two other seasons have peaked in March (2011-12 and 2005-06). Overall influenza activity was moderate this season, with a lower percentage of outpatient visits for influenza-like illness (ILI),(†) lower hospitalization rates, and a lower percentage of deaths attributed to pneumonia and influenza (P&I) compared with the preceding three seasons. Influenza A(H1N1)pdm09 viruses predominated overall, but influenza A(H3N2) viruses were more commonly identified from October to early December, and influenza B viruses were more commonly identified from mid-April through mid-May. The majority of viruses characterized this season were antigenically similar to the reference viruses representing the recommended components of the 2015-16 Northern Hemisphere influenza vaccine (1). This report summarizes influenza activity in the United States during the 2015-16 influenza season (October 4, 2015-May 21, 2016)(§) and reports the vaccine virus components recommended for the 2016-17 Northern Hemisphere influenza vaccines.

Update: Influenza Activity--United States, October 4, 2015-February 6, 2016.

From October through mid-December 2015, influenza activity remained low in most regions of the United States. Activity began to increase in late December 2015 and continued to increase slowly through early February 2016. Influenza A viruses have been most frequently identified, with influenza A (H3N2) viruses predominating during October until early December, and influenza A (H1N1)pdm09 viruses predominating from mid-December until early February. Most of the influenza viruses characterized during that time are antigenically similar to vaccine virus strains recommended for inclusion in the 2015-16 Northern Hemisphere vaccines. This report summarizes U.S. influenza activity* during October 4, 2015-February 6, 2016, and updates the previous summary (1).

Update: Influenza Activity - United States.

CDC collects, compiles, and analyzes data on influenza activity year-round in the United States. The influenza season generally begins in the fall and continues through the winter and spring months; however, the timing and severity of circulating influenza viruses can vary by geographic location and season. Influenza activity in the United States remained low through October and November in 2015. Influenza A viruses have been most frequently identified, with influenza A (H3) viruses predominating. This report summarizes U.S. influenza activity for the period October 4-November 28, 2015.

Update: Influenza Activity--United States and Worldwide, May 24-September 5, 2015.

During May 24­September 5, 2015, the United States experienced typical low levels of seasonal influenza activity. Influenza A (H1N1)pdm09 (pH1N1), influenza A (H3N2), and influenza B viruses were detected worldwide and were identified sporadically in the United States. All of the influenza viruses collected from U.S. states and other countries during that time have been characterized antigenically and/or genetically as being similar to the influenza vaccine viruses recommended for inclusion in the 2015­16 Northern Hemisphere vaccine. During May 24­September 5, 2015, three influenza variant† virus infections were reported; one influenza A (H3N2) variant virus (H3N2v) from Minnesota in July, one influenza A (H1N1) variant (H1N1v) from Iowa in August, and one H3N2v from Michigan in August.