Influenza activity in the United States was low during the summer months, and most of the detected strains match the 2017 to 2018 vaccine, the Centers for Disease Control and Prevention (CDC) has reported.
From late May through late June, influenza B viruses were predominant, whereas influenza A viruses were more common beginning in July, Lenee Blanton, MPH, from the Influenza Division, National Center for Immunization and Respiratory Diseases, CDC, Atlanta, Georgia, and colleagues write. They published their findings in the October 6 issue of the Morbidity and Mortality Weekly Report.
"The majority of the influenza viruses collected from the United States and other countries during that time were characterized antigenically and genetically as being similar to the cell-grown reference viruses representing the 2017 Southern Hemisphere and 2017–18 Northern Hemisphere influenza vaccine viruses," the researchers explain.
Of 159,828 respiratory specimens tested at clinical and public health laboratories in the United States from May 21 through September 23, 5321 were positive for influenza, including 2727 that were positive for influenza A viruses and 2594 that were positive for influenza B viruses. Of the influenza A viruses that were subtyped by public health laboratories, 86.4% were influenza A(H3N2) and 13.6% were influenza A(H1N1)pdm09. Of the influenza B viruses that were subtyped, 74.1% belonged to the B/Yamagata lineage and 25.9% belonged to the B/Victoria lineage.
According to the World Health Organization's recommendation for the northern hemisphere's 2017 to 2018 influenza season, trivalent vaccines should include A/Michigan/45/2015 (H1N1) pdm09-like virus, an A/Hong Kong/4801/2014 (H3N2)-like virus, and a B/Brisbane/60/2008-like (B/Victoria lineage) virus. Quadrivalent vaccines should add a B/Phuket/3073/2013-like (B/Yamagata lineage) virus.
Antigenic and genetic characterization of the circulating influenza strains showed low levels of A(H1N1) pdm09 viruses with no substantial genetic or antigenic changes, "even among viruses from regions that experienced higher A(H1N1)pdm09 activity," the authors write. Since early July, influenza A (H3N2) viruses have been predominant in the United States, with no evidence to date of a predominant subclade with substantial antigenic drift, despite extensive genetic variation in the circulating virus population.
Among the detected influenza B strains, two subgroups of antigenically distinct influenza B/Victoria viruses emerged, and although these represented a minority of B/Victoria viruses circulating globally, the authors advise close monitoring of them to assess potential public health implications. "Although influenza B viruses circulate throughout the influenza season, they frequently circulate later in the season than do influenza A viruses and often result in a second peak of influenza activity, often in the late winter and spring in the United States and other Northern Hemisphere countries," they write.
Multiple influenza vaccines are approved and recommended for use and are being distributed during the 2017 to 2018 season. With respect to the inactivated vaccines, "all influenza A(H1N1) and A(H3N2) and both influenza B components will be egg-derived, with the exception of ccIIV4, for which the influenza A(H3N2) virus component will, for the first time, be a cell-derived vaccine virus component," the authors explain. "This represents a first step toward producing a totally egg-independent inactivated virus vaccine."
The use of egg-independent vaccine technologies is notable because the resulting vaccines will likely be a more precise match for circulating viruses and potentially offer better protection, they add.
"Because of the low effectiveness of live attenuated intranasal influenza vaccine (LAIV4) against influenza A(H1N1)pdm09 viruses in the United States during the 2013–14 and 2015–16 seasons, for the 2017–18 season, the Advisory Committee on Immunization Practices and CDC renewed the recommendation that LAIV4 should not be used," the authors report.
Annual influenza vaccination, which the CDC recommends for all adults and children 6 months of age and older, offers the best protection for preventing and reducing influenza-related morbidity and mortality, the authors stress. Individuals with confirmed or suspected influenza who are at high risk for serious complications should receive early treatment with antiviral medication.
One coauthor reports a US patent issued January 2, 2001, for preparation and use of recombinant influenza A virus M2 construct vaccine, and a US patent issued April 26, 2012, for an effective vaccine against pandemic strains of influenza viruses. The remaining authors have disclosed no relevant financial relationships.
Morb Mortal Wkly Rep. Published online October 6, 2017. Full text
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