Influenza Activity — United States and Worldwide, May 19–September 28, 2019, and Composition of the 2020 Southern Hemisphere Influenza Vaccine

Scott Epperson, DVM; C. Todd Davis, PhD; Lynnette Brammer, MPH; Anwar Isa Abd Elal; Noreen Ajayi, MPH; John Barnes, PhD; Alicia P. Budd, MPH; Erin Burns, MA; Peter Daly, MPH; Vivien G. Dugan, PhD; Alicia M. Fry, MD; Yunho Jang, PhD; Sara Jo Johnson, MPH; Krista Kniss, MPH; Rebecca Kondor, PhD; Lisa A. Grohskopf, MD; Larisa Gubareva, PhD; Angiezel Merced-Morales, MPH; Wendy Sessions, MPH; James Stevens, PhD; David E. Wentworth, PhD; Xiyan Xu, MD; Daniel Jernigan, MD


Morbidity and Mortality Weekly Report. 2019;68(40):880-884. 

In This Article


From May to September 2019, influenza activity remained low in the United States, as is typical for that time of year. Influenza A and B viruses cocirculated throughout the summer months with influenza A(H3N2) viruses predominating overall and influenza B/Victoria, subclade V1A-3Del, viruses the most common influenza B virus reported by public health laboratories. Influenza A and B viruses also circulated widely in the Southern Hemisphere with the predominant virus varying by region and country. It is too early in the season to know which viruses will circulate in the United States later this fall and winter or how severe the season might be; however, regardless of what is circulating, the best protection against influenza is an influenza vaccination. Influenza vaccination has been shown to reduce the risk for influenza illness associated with outpatient health care visits and hospitalizations and reduces the risk for serious influenza outcomes that can result in hospitalization or death. CDC recommends that all persons aged 6 months and older who do not have contraindications get vaccinated, but vaccination is especially important for persons at high risk for serious influenza-associated complications, including persons aged ≥65 years, children aged <5 years, pregnant women, and persons with certain underlying medical conditions.

In late September, WHO issued its recommendations for the 2020 Southern Hemisphere influenza vaccine. Compared with the composition of the 2019–20 Northern Hemisphere influenza vaccine formulation, these recommendations reflect changes to the A(H3N2) and B/Victoria-lineage components. The update for the B/Victoria-lineage component reflects the global spread and increase of V1A-3Del viruses, which had reduced reactivity to ferret antisera raised to V1A.1 viruses used in 2019–20 Northern Hemisphere vaccines. Apart from North and South America, the majority of A(H3N2) viruses circulating elsewhere globally belonged to subclade 3C.2a1 and were antigenically different from the Northern Hemisphere 3C.3a vaccine component, leading to a change in the A(H3N2) component to a 3C.2a1 subclade virus for the Southern Hemisphere. These recommendations were made specifically for the Southern Hemisphere using many factors, including evolutionary approaches to forecast specific subgroups likely to circulate 6 months into the future, determining which candidate vaccine viruses induce immunity that blocks the largest variety of viruses and which viruses escape population immunity from prior infection or vaccination. These factors vary among countries within the Southern Hemisphere and certainly vary between the Southern and Northern Hemispheres. For example, activity in Australia during recent seasons has not reflected influenza virus activity in the subsequent U.S. season. Changes to the Southern Hemisphere vaccine composition, therefore, might not be a good predictor of the upcoming U.S. influenza season. Although Australia experienced an early start to its 2019 season with influenza A(H1N1)pdm09 viruses circulating initially and A(H3N2) virus eventually predominating,[2] influenza is unpredictable, and circumstances can change very quickly. Analysis of surveillance and laboratory data to date continues to support the appropriateness of the Northern Hemisphere vaccine viruses used in production of influenza vaccines for the upcoming U.S. season.

Except for one influenza A(H1N1)pdm09 virus and one influenza B virus, all influenza viruses tested remained susceptible to oseltamivir, peramivir, and zanamivir, and only one virus contained a genetic mutation that has previously been associated with reduced susceptibility to baloxavir. Influenza antiviral medications are a valuable adjunct to annual influenza vaccination, and early treatment with influenza antiviral medication, especially within 48 hours of symptom onset, is recommended for patients with confirmed or suspected influenza who 1) have severe, complicated, or progressive illness; 2) require hospitalization; or 3) are at high risk for influenza-related complications§§.[3] Early treatment has been shown to decrease time to symptom improvement[4–7] and to reduce secondary complications associated with influenza.[8,9] Health care providers should not delay treatment until test results become available because treatment is most effective when given early in the illness. Additional information regarding influenza viruses, influenza surveillance, influenza vaccines, influenza antiviral medications, and novel influenza A virus infections in humans is available at https://www.cdc.gov/flu.

§§ Persons at high risk include 1) children aged <5 years; 2) adults aged ≥65 years; 3) persons with chronic pulmonary conditions (including asthma), cardiovascular disease (except hypertension alone), renal, hepatic, hematologic (including sickle cell) disease, metabolic disorders (including diabetes mellitus), or neurologic and neurodevelopmental conditions (including disorders of the brain, spinal cord, peripheral nerves, and muscles, such as cerebral palsy, epilepsy [seizure disorders], stroke, intellectual disability [mental retardation], moderate to severe developmental delay, muscular dystrophy, or spinal cord injury); 4) persons with immunosuppression, including that caused by medications or by human immunodeficiency virus infection; 5) women who are pregnant or postpartum (within 2 weeks after delivery); 6) persons aged ≤18 years who are receiving long-term aspirin therapy; 7) American Indians/Alaska Natives; 8) persons with extreme obesity (i.e., body mass index ≥40); and 9) residents of nursing homes and other chronic care facilities.