- Like all flu seasons, mortality in the elderly was highest from swine flu during the 2009-10 season
- Preexisting antibodies were common in the elderly: present in 88% of those aged 90-99, present in 27% of those aged 79, and present in 4-5% in those younger than 60.
- Six studies from Canada consistently found that prior year vaccination in 2008/09 for seasonal influenza was associated with a 1.4- to 2.5-fold increased risk for hospitalization for H1N1 infection, said Naveed Z. Janjua, MD, also from the British Columbia Centre for Disease Control.
"As for the mechanism behind those differences, studies in swine suggest that a vaccine that induces "nonneutralizing or subneutralizing concentrations of antibody can enhance the infection rather than protect from that infection," Dr. Janjua said.
Dr. Skowronski acknowledged the limitations of case–controlled studies. She suggested that the Canadian findings might be more rigorous than similar studies from the United States and Mexico that reached different conclusions on the effect of vaccination for seasonal influenza on H1N1 infection.
Good immunization records and the use of a single brand of vaccine within a geographic region also are factors contributing to the rigor of the Canadian analysis, she pointed out.
The studies were conducted with funding from various Canadian government agencies. None of the presenters disclosed no relevant financial relationships."