Update: Influenza Activity — United States and Worldwide, May 20–October 13, 2018

Eric J. Chow, MD; C. Todd Davis, PhD; Anwar Isa Abd Elal; Noreen Alabi, MPH; Eduardo Azziz-Baumgartner, MD; John Barnes, PhD; Lenee Blanton, MPH; Lynnette Brammer, MPH; Alicia P. Budd, MPH; Erin Burns, MA; William W. Davis, DrPH; Vivien G. Dugan, PhD; Alicia M. Fry, MD; Rebecca Garten, PhD; Lisa A. Grohskopf, MD; Larisa Gubareva, PhD; Yunho Jang, PhD; Joyce Jones, MS; Krista Kniss, MPH; Stephen Lindstrom, PhD; Desiree Mustaquim, MPH; Rachael Porter, MPH; Melissa Rolfes, PhD; Wendy Sessions, MPH; Calli Taylor, MPH; David E. Wentworth, PhD; Xiyan Xu, MD; Natosha Zanders, MS; Jacqueline Katz, PhD; Daniel Jernigan, MD


Morbidity and Mortality Weekly Report. 2018;67(42):1178-1185. 

In This Article


In the United States, ILI activity remained below baseline levels during May 20–October 13, 2018; low levels of laboratory-confirmed influenza were reported as a result of a mix of influenza A and B. In the Southern Hemisphere, low levels of influenza activity were observed with a predominance of A(H1N1)pdm09 viruses. Analysis of available viruses suggests that minimal drift of viruses has occurred.

Vaccination before the onset of influenza activity is the primary strategy to prevent influenza-associated illness and its potentially serious complications. A recent report indicated the high prevalence of influenza illnesses resulted in approximately 79,000 deaths and 960,000 hospitalizations during the 2017–18 influenza season (https://www.cdc.gov/flu/about/disease/burden.htm). Influenza vaccination prevents millions of medical visits, tens of thousands of hospitalizations, and thousands of deaths each year, even with vaccine effectiveness estimates in the range of 40%–60%. Health care providers should urge their patients to get vaccinated by the end of October, if they have not already been vaccinated. Vaccination efforts should continue throughout the influenza season.

In late September, WHO issued its recommendations for the 2019 Southern Hemisphere influenza vaccine. Surveillance has shown that there has been no significant evidence of antigenic drift among circulating A(H3N2) viruses since the selection of viruses for the 2018–19 Northern Hemisphere vaccines was made in February. However, the influenza A(H3N2) component for egg-based vaccines was updated to address genetic and antigenic changes that occur when A(H3N2) vaccine viruses are propagated in eggs. The A(H3N2) component was updated because sera against egg-propagated A/Switzerland/8060/2017 (H3N2) virus showed better reactivity with an increasing number of circulating A(H3N2) viruses than sera generated against egg-propagated A/Singapore/INFIMH-16-0019/2016. No changes were recommended for the A(H3N2) component of cell-manufactured or recombinant vaccines. It is difficult to predict which influenza virus will predominate or what the season will be like, but there will likely be cocirculation of influenza A(H1N1), A(H3N2), and B influenza viruses.

Annual influenza vaccination is the best method for preventing influenza infection and its potentially serious complications. In the United States, annual influenza vaccine is recommended for all persons aged ≥6 months who do not have a contraindication.[1] Influenza vaccination has been shown to reduce the risk for influenza illness, and a growing body of evidence suggests that vaccination also reduces the risk for serious influenza outcomes that can result in hospitalization and even death. A CDC study in 2017 showed influenza vaccination reduced the risk for influenza-associated death by 51% among children with underlying high-risk medical conditions and by 65% among healthy children.[2] Most recently, an August 2018 study showed that influenza vaccination lessened the risk for severe influenza among adults, including reducing the risk for hospitalization and admission to the intensive care unit, and also lessened severity of illness (https://www.cdc.gov/flu/spotlights/vaccine-reduces-risk-severe-illness.htm). These benefits are especially important for persons at high risk for serious influenza complications, including persons aged ≥65 years, children aged <5 years, pregnant women, and persons with certain underlying long-term medical conditions, including heart and lung disease, or diabetes.

Ideally, influenza vaccination should be administered before the start of community influenza activity. However, health care providers should continue to offer annual influenza vaccine to unvaccinated persons as long as influenza viruses continue to circulate. For the 2018–19 influenza season, multiple influenza vaccines are approved and recommended for use; there is no preferential recommendation for one influenza vaccine product over another for persons for whom more than one is suitable.[1] Children aged 6 months–8 years require 2 doses of influenza vaccine administered ≥4 weeks apart if they have not received at least 2 doses of influenza vaccine before July 1, 2018.[3] Those who have previously received at least 2 doses before this date only require a single dose for 2018–19, even if the 2 previous doses were not received during the same or consecutive seasons.[1] For the 2018–19 season, interim supply projections by manufacturers for the U.S. market range from 163 to 168 million doses of influenza vaccine.

Influenza antiviral medications can serve as a valuable adjunct to annual influenza vaccination. Early treatment with influenza antiviral medication is recommended for patients with confirmed or suspected influenza who have severe, complicated, or progressive illness; who require hospitalization; or who are at high risk for influenza-related complications.¶¶¶¶ Early treatment has been shown to decrease time to symptom improvement[4–7] and to reduce secondary complications associated with influenza.[8,9] Providers should not delay treatment until test results become available because treatment is most effective when given early in the illness, especially within 48 hours of symptom onset.[10] Providers should also not rely on less sensitive assays such as rapid antigen detection influenza diagnostic tests to inform treatment decisions.[10]

During May 20–October 13, fewer human infections with variant viruses were reported compared with most previous seasons.***** Most of these variant viruses were influenza A(H1N2)v viruses, and A(H1N2) viruses have predominated in swine in some regions of the United States.††††† All but two of the patients with variant virus infections reported swine exposure and attendance at an agricultural fair; one only attended an agricultural fair, and another reported neither swine exposure nor attendance at an agricultural fair. Vulnerable populations, especially young children and other persons at high risk for serious influenza complications, should avoid swine barns at agricultural fairs or close contact with swine. Health care providers should consider novel influenza virus infections in persons with ILI and swine or poultry exposure, or with severe acute respiratory infection after travel to areas where avian influenza viruses have been detected.

Influenza surveillance reports for the United States are posted online weekly and are available at https://www.cdc.gov/flu/weekly. 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 <2 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.
††††† https://www.aphis.usda.gov/animal_health/animal_dis_spec/swine/downloads/fy2018quarter1swinereport.pdf.