CDC Researchers Highlight Lessons Learned from H3N2v, July 2011-April 2012

Findings from a number of CDC studies related to H3N2v were recently released in the journal Clinical Infectious Diseases. These epidemiologic and modeling studies look back at the first cases of H3N2v detected in 2011, but offer ongoing relevant insight into the ongoing 2013 H3N2v outbreaks.

Key lessons learned during the 2011 “first wave” of H3N2v cases:

Children are at highest risk of infection with H3N2v.

Based on epidemiology of the first 13 cases, Epperson et al conclude that children appear to be at highest risk of infection with H3N2v, which is consistent with results of limited serologic studies indicating that young children have little or no preexisting immunity to H3N2v, while low levels of cross-reactive antibody have been detected in older children and adults.

Additionally, several of the 13 cases were identified in the context of day care outbreaks, which suggests an increase of limited person-to-person transmission of H3N2v compared to previously reported infections with variant influenza viruses where swine exposure was more frequent.

Many H3N2v infections likely went undetected.

Biggerstaff et al developed a mathematical model that suggests H3N2v infections were more frequent than previously thought from August 2011 to April 2012. Based on the model, the authors estimate that only about 1 in 200 H3N2v infections were reported, indicating that many more likely occurred than were detected.

CDC uses existing influenza surveillance data and informed assumptions to produce mathematical models. In this instance, CDC scientists used the number of reported H3N2v cases to estimate the actual number of cases that might have occurred given that most persons with influenza-like illness (ILI) do not seek medical care, many doctors do not perform laboratory testing of patients with ILI, and most laboratories would not have been able to detect the virus because polymerase chain reaction testing for this virus was only available at a few laboratories. To account for these discrepancies, researchers adapted a multiplier model created at the beginning of the 2009 H1N1 pandemic to estimate the true burden of H3N2v illness.

H3N2v is mostly associated with prolonged exposure to pigs at agricultural fairs.

Wong et al developed a model to estimate the probability of H3N2v transmission from pigs to humans, and conclude that although the risk of H3N2v virus infection is small for fair attendees with minimal swine contact, large populations attend agricultural events each year, and human cases will likely occur when infected swine are present.

Several factors contribute to the age distribution of H3N2v cases.

Gambhir et al developed a mathematical model of an influenza outbreak to illustrate how the age distribution of H3N2v cases is dependent on several factors including age-specific variations in exposure to and susceptibility to H3N2v infection, and attributing age distribution of cases to prior immunity alone should be avoided. The model includes both zoonotic and human-to-human routes of transmission and shows how the age distribution changes according to the relative importance of each of these routes.

To date, the findings from the epidemiologic and modeling studies of the 2011 cases have held true in 2012 during a much larger outbreak with 306 confirmed cases in 10 states, associated with 37 fairs. Most of the 2012 cases have been in children, most have been associated with swine exposure, and there has been very little, if any, human-to-human transmission. The epidemiology of the recent 2013 H3N2v outbreak in Indiana so far seems consistent with what was seen last summer.


Since the mid-1990s, modeling has been an important element in CDC’s pandemic influenza preparedness activities. Models provide insight and can inform public health strategies and policy decisions by illustrating the impact that public health interventions may have on mitigating influenza illness.

According to authors Dr. Lyn Finelli and Dr. David Swerdlow, if initial H3N2v modeling studies had not been performed, concerns at the onset of the large H3N2v outbreak in summer 2012 could have led to implementation of unnecessary and costly control measures.

The supplement is available online from the Clinical Infectious Diseases website.