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

Novel Influenza a Virus Infections

Fourteen human infections with novel influenza A viruses were reported in the United States during May 20–October 13. Influenza viruses that normally circulate in swine and not humans are called "variant" viruses when detected in humans and designated with the letter v after the subtype. One infection was associated with an influenza A(H3N2)v virus, and 13 were associated with influenza A(H1N2)v viruses. All but one infection occurred among persons aged <18 years. The A(H3N2)v virus infection was reported from Indiana in a patient who reported swine contact at an agricultural fair in the week before symptom onset. All A(H1N2)v virus infections were reported in August from three states: California (six cases), Ohio (four), and Michigan (three). Eleven of the 13 patients reported contact with swine at agricultural fairs, one reported attendance at an agricultural fair but no contact with swine, and one reported neither contact with swine nor attendance at an agricultural fair. Limited human-to-human transmission might have taken place with this last A(H1N2)v infection; however, no ongoing or sustained human-to-human transmission associated with any of these infections was identified. None of the novel influenza A virus infections resulted in hospitalization, and all patients recovered.

The genome of the one A(H3N2)v virus(A/Indiana/27/2018) was closely related to A(H3N2)v viruses detected during 2017 and viruses known to circulate in the U.S. swine population. Antigenic testing showed reduced inhibition by ferret antisera raised to the nearest A(H3N2)v candidate vaccine virus (CVV), but postvaccination antisera from adults vaccinated with the 2017–18 influenza vaccine reacted with the virus at titers that were within fourfold of those against the homologous reference virus, A/Michigan/15/2014, representing the A(H3N2) component of the 2017–18 seasonal influenza vaccines. Postvaccination sera collected from children, however, had lower titers to this virus than to the A/Michigan/15/2014 homologous virus titer. These studies indicate that vaccination with the 2017–18 seasonal influenza vaccine might offer less protection against this A(H3N2)v virus for children than adults.

All of the A(H1N2)v viruses detected had HA gene segments from the delta 2 sublineage of the swine influenza virus H1 HA lineage. The HA and neuraminidase gene segments of these viruses were closely related to 2017 and 2018 A(H1N2) influenza viruses circulating in the U.S. swine population, including swine identified at the agricultural fairs attended by infected persons and viruses sporadically detected in previous A(H1N2)v zoonotic infections. Antigenic testing demonstrated that all of the 2018 A(H1N2)v viruses were well inhibited by ferret antisera raised to the nearest CVV. HI reactivity of pooled, child and adult postvaccination antisera from persons vaccinated with the 2017–18 vaccine was below the limit of detection for all viruses tested. These studies indicate that vaccine viruses specially developed to prevent A(H1N2)v virus infections would be protective; however, vaccination with the seasonal vaccine would not offer any protection.