By Tom Rosenthal

ATLANTA—The CDC’s Advisory Committee on Immunization Practices (ACIP) voted unanimously not to change its recommendation of an annual influenza vaccination for all people aged 6 months and older who do not have contraindication to the vaccine.

The components for the trivalent vaccine for the 2019-2020 influenza season were also announced at the meeting:

  • A/Brisbane/02/2018 (H1N1)pdm09-like virus—an update;
  • A/Kansas/14/2017 (H3N2)-like virus—an update; and
  • B/Colorado/06/2017-like virus (Victoria lineage).

Quadrivalent vaccines will include the above three components plus B/Phuket/3073/2013-like virus (Yamagata lineage).

The selection of the H3N2 component was delayed until March 21 to enable the CDC to acquire sufficient data to address the unusual nature of the 2018-2019 flu season, Lynnette Brammer, MPH, the head of the CDC’s Domestic Influenza Surveillance team, told the committee.

“This season was remarkable because we had two roughly equal waves of influenza A viruses,” Ms. Brammer said. The initial outbreak was influenza A(H1N1) viruses followed by influenza A(H3N2) viruses, which became dominant during the last week of February 2019, she said. There were very little influenza B viruses this flu season, she noted.

Brendan Flannery, MD, the lead investigator for the U.S. Flu Vaccine Effectiveness Network, told the committee that the vaccine had an effectiveness of approximately 47% for the A(H1N1) viruses during the start of the season. However, he said, the vaccine’s effectiveness dropped to approximately 9% with the A(H3N2) viruses during the second wave.

While the vaccine’s overall effectiveness was approximately 30%, it prevented 40,000 to 90,000 hospitalizations, Dr. Flannery said.

The outbreak lasted 21 weeks, “the longest period that we’ve seen in the last 10 years, probably due to the two waves of influenza A virus,” Ms. Brammer said. The flu season peaked at mid-February, she said. Hospitalization rates were 65.1 lab-confirmed influenza-related hospitalizations per 100,000 population, affecting most heavily those older than 65 years of age.

The CDC classified the 2018-2019 flu season as moderate for all age groups, Ms. Brammer said. There were 37.4 million reported influenza illnesses, of which 17.3 million were medically attended, at least 531,000 hospitalizations, and at least 36,400 deaths, she said. There were 119 pediatric influenza deaths, lower than the 2017-2018 season but higher than other recent years, she said.

The manufacturers of influenza vaccines said despite the delay in announcing the components for the trivalent vaccine, they will be able to meet the anticipated need of 162 million to 169 million flu shots, Lisa Grohskopf, MD, MPH, who heads the ACIP Influenza Work Group, told the ACIP members.

Influenza is constantly being monitored because the flu season is different in various regions. The Southern Hemisphere, which is just heading into winter, is already seeing influenza, and the Australian Department of Health said influenza and influenza-like illness activity are higher this time of year compared with previous years.

Several members of the committee asked whether these reports give the United States any indication about its coming influenza season. 

In response to a question regarding flu season outbreaks in Australia, Ms. Brammer said the CDC receives many questions about that country’s flu seasons as an indication of America’s. She said the CDC watches Australia carefully to understand the viruses there. However, she said, “Some years we do have a season similar to Australia and some years we don’t.”

The ACIP’s recommendations are reviewed by the CDC director and the Department of Health and Human Services. Final recommendations are official when published in Morbidity and Mortality Weekly Report.