Illness during Spring-Summer 2009: Four Observational Studies from Canada
http://www.plosmedicine.org/article/info%3Adoi%2F10.1371%2Fjournal.pmed.1000258
Background
In late spring 2009, concern was raised in Canada that prior vaccination
with the 2008-09 trivalent inactivated influenza vaccine (TIV) was
associated with increased risk of pandemic influenza A (H1N1) (pH1N1)
illness. Several epidemiologic investigations were conducted through the
summer to assess this putative association.
Methods and Findings
Studies included: (1) test-negative case-control design based on Canada's
sentinel vaccine effectiveness monitoring system in British Columbia,
Alberta, Ontario, and Quebec; (2) conventional case-control design using
population controls in Quebec; (3) test-negative case-control design in
Ontario; and (4) prospective household transmission (cohort) study in
Quebec. Logistic regression was used to estimate odds ratios for TIV effect
on community- or hospital-based laboratory-confirmed seasonal or pH1N1
influenza cases compared to controls with restriction, stratification, and
adjustment for covariates including combinations of age, sex, comorbidity,
timeliness of medical visit, prior physician visits, and/or health care
worker (HCW) status. For the prospective study risk ratios were computed.
Based on the sentinel study of 672 cases and 857 controls, 2008-09 TIV was
associated with statistically significant protection against seasonal
influenza (odds ratio 0.44, 95% CI 0.33-0.59). In contrast, estimates from
the sentinel and three other observational studies, involving a total of
1,226 laboratory-confirmed pH1N1 cases and 1,505 controls, indicated that
prior receipt of 2008-09 TIV was associated with increased risk of medically
attended pH1N1 illness during the spring-summer 2009, with estimated risk or
odds ratios ranging from 1.4 to 2.5. Risk of pH1N1 hospitalization was not
further increased among vaccinated people when comparing hospitalized to
community cases.
Conclusions
Prior receipt of 2008-09 TIV was associated with increased risk of medically
attended pH1N1 illness during the spring-summer 2009 in Canada. The
occurrence of bias (selection, information) or confounding cannot be ruled
out. Further experimental and epidemiological assessment is warranted.
Possible biological mechanisms and immunoepidemiologic implications are
considered.
