A Summary of the 2018-19 US Flu Season

The 2018-2019 United States flu season was the longest season in a decade, the US Centers for Disease Control and Prevention (CDC) has announced.

Influenza-like illness activity began increasing November and peaked during mid-February, returning to below baseline in mid-April. In total, the season lasted 21 weeks.

The report, published in the CDC’s Morbidity and Mortality Weekly Report, notes that although the United States experienced a lengthy flu season, overall the season was of moderate severity.

Illnesses attributed to influenza A viruses predominated, with very little influenza B activity. According to surveillance data, among 80,993 specimens tested at public health laboratories throughout the season, 42,303 (52.2%) were positive for influenza viruses—40,624 (96.0%) were positive for influenza A and 1679 (4%) for influenza B.

Subtype information was available for 38,995 cases of influenza A, with 22,084 (56.6%) cases of A(H1N1)pdm09 and 16,991 (43.6%) cases of A(H3N2). The report documents that there were 2 notable waves of influenza A activity: from October to mid-February there was an influx of the A(H1N1)pdm09 cases and from mid-February to mid-May there was an increase in A(H3N2) cases.

Influenza B subtypes were available for 1105 (65.8%) influenza B viruses; 406 (36.7%) were B/Yamagata lineage; and 699 (63.3%) were B/Victoria lineage.

Most of the A(H1N1)pdm09 viruses were antigenically similar to the cell culture-propagated reference virus representing the 2018-2019 Northern Hemisphere influenza vaccine virus, but considerable genetic diversity among currently circulating A(H1N1)pdm09 viruses belonging to clade 6B,1A was observed. Additionally, the increased circulation of clade 3C.3a viruses strongly contributed to the increasing proportion of A(H3N2) viruses that were antigenically distinct from the reference virus representing the A(H3N2) component of the 2018-2019 Northern Hemisphere vaccines.

The investigators also noted that the severity assessment was moderate across all age groups. The rate of hospitalization was lower for adults compared with the 2017-2018 influenza season, but similar for children. Between September 30, 2018, and May 18, 2019, 116 laboratory-confirmed influenza-associated pediatric deaths were reported.

More than 99% of influenza virus tested since October 1, 2018, were susceptible to oseltamivir and peramivir, and all tested viruses were susceptible to zanamivir and baloxavir.

The CDC warns that although the United States has entered the summer months and seasonal influenza activity is currently below baseline, influenza illnesses are often reported outside of flu season.

Influenza should be considered as a possible diagnosis in ill travelers returning from countries with ongoing influenza activity. When variant influenza infection is suspected, clinicians should make referrals to state health departments for testing.

The authors of the report imply that receiving a seasonal influenza vaccine each year remains the best way to protect against seasonal influenza and its potentially severe consequences.

Vaccine recommendations for the 2019-20 influenza season as made by the World Health Organization and US Food and Drug Administration’s Vaccines and Related Biologic Products Advisory Committee suggest “that influenza trivalent vaccines contain an A/Brisbane/02/2018 A(H1N1)pdm09-like virus, an A/Kansas/14/2017 A(H3N2)-like virus, and a B/Colorado/06/2017-like (B/Victoria lineage) virus. The quadrivalent vaccine recommendation included the trivalent vaccine viruses and a B/Phuket/3073/2013-like (B/Yamagata lineage) virus.”