Current CDC data indicate that seasonal vaccines may provide limited protection against infection with influenza A(H3N2)v viruses among adults, but will afford no protection in children.
Since August 2011, 29 Americans have been infected with influenza A variant virus H3N2v, the CDC reports. Investigations revealed human infections with these viruses following contact with swine as well as limited human-to-human transmission. The following states have reported cases: Hawaii (1), Indiana (7), Iowa (3), Maine (2), Ohio (10), Pennsylvania (3), Utah (1), and West Virginia (2).
While H3N2v viruses have been detected in U.S. swine, it’s unknown how widespread they are in swine herds. It’s possible that sporadic infections and even localized outbreaks among people with this virus will continue to occur. While there is no evidence that sustained human-to-human transmission is occurring, all influenza viruses have the capacity to change and it’s possible that this virus may become widespread, the CDC notes. So far, the severity of illnesses associated with this virus in people has been similar to the severity of illnesses associated with seasonal flu virus infections. Limited serologic studies indicate that adults may have some pre-existing immunity to this virus while children do not.
Most infections with this virus have resulted in self-limited, mild respiratory illnesses; however, three persons have been hospitalized. None have died. While it is unknown whether this virus will continue to occur among humans or become more common, it is possible that health care providers will care for patients that are infected with this virus. There are no data to indicate that the transmission characteristics of the A(H3N2)v virus will be different than those of seasonal influenza viruses. As a result, CDC advises that the infections control principles and actions relevant for seasonal influenza are appropriate for the control of A(H3N2)v as well.
Of special note is that the current infection control guidance recommends vaccination of health care workers and patients as a critical step to reduce seasonal influenza transmission in these settings. While the effectiveness of current seasonal vaccines to protect against A(H3N2)v virus infections might be reduced compared with effectiveness of seasonal vaccines against seasonal influenza, CDC recommends their use. They remain the best tool for the prevention of seasonal influenza transmission in health care settings, which is currently the greatest risk from influenza during this influenza season.