CDC offers theory on H1N1 and resistance.

Results from a Centers for Disease Control and Prevention study are yielding more concrete clues as to why the novel influenza A (H1N1) virus seems to be striking people aged 5-25 years in greater numbers than those aged 60 years and older. The agency has also concluded that recent seasonal influenza vaccines are "unlikely" to protect people, regardless of age, from the novel virus.

According to a CDC report released in May, one possible explanation for this seemingly higher resistance to the virus in people aged 60 years and older is that adults in this age group have had previous exposure, "either through infection or vaccination, to an influenza A (H1N1) virus that is genetically and antigenically more closely related to the novel influenza A (H1N1) virus than are con-temporary seasonal H1N1 strains."

A collaborative effort, this study--released in the CDC's Morbidity and Mortality Weekly Report--involved researchers from a range of institutions including the Food and Drug Administration's Center for Biologics Evaluation and Research, the National Institute of Allergy and Infectious Diseases, three universities, and two pharmaceutical companies.

Using stored serum specimens collected during previous vaccine studies (from healthy, human participants), the CDC assessed the level of cross-reactive antibody to H1N1 in cohorts of children and adults before and after vaccination with seasonal influenza vaccines from 2005-2006 to the most recent 2008-2009 vaccine.

As done in vaccine production, the seasonal influenza A (H1Nl) viruses used in the study were propagated in embryonated chicken eggs, and the study's novel influenza A (HlNl) virus was grown in Mandin-Darby canine kidney cells.

The researchers found that no cross-reactive antibody to the novel influenza A (H1N1) virus existed among children.

However, among adults before vaccination, cross-reactive antibody was found in 6%-9% of those aged 18-64 years and 33% in people aged 60 years and older.

The investigators also analyzed response in children who had any of four seasonal trivalent, inactivated influenza vaccines (TIV) or live, attenuated influenza vaccine (LAIV).

Again, children vaccinated with TIV or LAIV had no protective response to the novel influenza A (H1N1).

Despite the small number of sera from children rested in this analysis (n=28), the researchers were able to conclude that "US. children are largely serologically naive to the novel influenza A (H1N1) virus and that vaccination with seasonal TIV or LAIV does not elicit any measurable level of cross-reactive antibody to the novel virus."

Although the researchers did find that vaccination of adults with seasonal TIV did generally result in a small increase of antibodies against the novel influenza A (H1N1) virus, "whether such levels of cross-reactive antibody provide any protection against infection with novel influenza A (H1N1) virus is unknown."

Adults aged 18-64 years vaccinated with TIV showed only "a twofold increase in cross-reactive antibody response to the novel influenza A (H1N1) ... compared with a twelve-fold to nine-teen-fold increase in cross-reactive antibody response to the seasonal H1N1 strain," researchers wrote.

The CDC said the study suggests that, "The receipt of recent (2005-2009) seasonal influenza vaccines is unlikely to elicit a protective antibody response to the novel influenza A (H1N1) virus."

The CDC also said ongoing assessment of cross-reactive antibody response among persons in different age groups could "identify a particular age group that would allow further clarification of the cross-reactive serologic response."

Adults aged 60 and older have had previous exposure to an H1N1 virus that is more closely related to the novel flu virus than are 'contemporary seasonal H1N1 strains.'