Wisconsin and many other states are experiencing a late-season second wave of influenza A/H3N2 activity. Current circulating H3N2 viruses can be divided into 3 main groups: 3C.2a, 3C.2a1 and 3C.3a

The currently circulating 3C.2a and 3C.2a1 viruses are similar to the H3N2 component in the 2018-2019 influenza vaccine. The currently circulating 3C.3a viruses are different from the H3N2 component in the 2018-2019 influenza vaccine.

During this influenza season the proportion of H3N2 viruses that are 3C.3a has been increasing. As of week 11 (week ending March 16, 2019), 3C.3a viruses account for 65% of all H3N2 viruses identified this season in the United States. In Wisconsin, all H3N2 viruses submitted to the CDC since January 1, 2019 were identified as 3C.3a.

Because 3C.3a viruses are different from the 2018-2019 H3N2 vaccine component, this likely will have negative implications for vaccine effectiveness (VE) against this virus. CDC’s 2018-2019 interim estimate for VE against all H3N2 viruses this season was 44% as of data through February 2, 2019. VE against H3N2 viruses is likely to decrease as the proportion of 3C.3a viruses has increased.

CDC recommends prompt antiviral treatment of people with confirmed or suspected influenza infection, who:

  • are hospitalized
  • have severe, complicated or progressive illness
  • are at high risk of serious flu complications who develop flu symptoms, including people 65 years and older and residents of nursing homes and other long term care facilities

Starting treatment with an antiviral drug soon after flu symptoms begin may reduce the risk of some complications and development of more severe illness that could result in a hospital stay. Studies show that flu antiviral drugs work best for treatment when they are started within 48 hours of getting sick, but starting them later can still be helpful, especially if the sick person has a high-risk health condition or is very sick from flu.

Please see the CDC recommendations for influenza antiviral medication for more information.