CDC reports flu season in full swing and ruled by influenza B

Flu season is in full swing according to the latest data from the Centers for Disease Control and Prevention (CDC).¹ The weekly surveillance indicates that activity is high, but the marks of severity have not yet reached a high point this season. More than 63,000 positive specimens have been identified since tracking for the season began in September 2019, with 68.4% of the samples testing positive for influenza B/Victoria.

The first week of 2020 surveillance from clinical laboratories nationwide included 11,459 positive samples, which account for 17.9% of positive specimens in the season so far. Influenza type B also remains the more prominent form in the first week of 2020 with 7257 positive specimens and influenza A(H1N1) being found in 4202. Further subtyping shows that influenza B/Victoria was more prevalent than influenza B/Yamagata and that influenza A(H1N1)pdm09 was the most common subtype of influenza A found in week 1, which follows the general trend for the season as well.

Influenza activity was moderate or high in 39 states as well as in Washington, DC, and Puerto Rico for week 1 of surveillance ending January 4, 2020. Delaware, Idaho, and the US Virgin Islands had insufficient data for week 1. New Hampshire had minimal activity, and activity was considered low in Florida, Hawaii, Missouri, Montana, Nevada, Ohio, Vermont, and Wyoming. However, the disease was considered widespread in nearly every state. Epidemiologists called the spread only regional in Mississippi, North Dakota, and Vermont; only local in Washington, DC, and Hawaii; and only sporadic in the US Virgin Islands.

Hospitalizations are common in pediatric cases

Children aged 0 to 4 years have the second-highest rate of hospitalization for influenza this season, with a cumulative rate of 26.8 per 100,000 population. Children aged 5 to 17 years had a hospitalization rate of 8.3 per 100,000 population. For children who were hospitalized for influenza and had information on potential associated medical conditions, 46.1% were found to have at least 1 underlying condition with cardiovascular disease, metabolic disorder/obesity, and chronic lung disease/asthma being the top conditions reported.

There have been 32 deaths attributed to influenza in the pediatric population so far this flu season. Twenty-one deaths were linked to influenza B viruses. Further study of lineage was carried out on samples from 5 of the cases and all were found to be influenza B/Victoria viruses. Influenza A was linked to the other 11 deaths and subtyping was carried out on 6, which confirmed that all were A(H1N1)pdm09.

The CDC confirmed 116 influenza-associated pediatric deaths during the 2018-2019 flu season that spanned September 30, 2018, through May 18, 2019, the longest flu season in the previous 10 years.²

References:

1.    Centers for Disease Control and Prevention (CDC). Weekly U.S. Influenza Surveillance Report. Key updates for Week 1, ending January 4, 2020. CDC website. Available at: https://www.cdc.gov/flu/weekly/index.htm. Updated January 4, 2020. Accessed January 16, 2020.

2.    Centers for Disease Control and Prevention (CDC). Update: Influenza activity in the United States during the 2018-19 season and composition of the 2019-20 influenza vaccine. MMWR Morb Mortal Wkly Rep. 2019;68(24);544-551. Available at: https://www.cdc.gov/mmwr/volumes/68/wr/mm6824a3.htm?s_cid=mm6824a3_w. Accessed January 16, 2020.