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October 08, 2009

Seasonal Flu Vaccine May Help Protect Against H1N1

Laurie Barclay, MD
October 7, 2009 — Seasonal influenza vaccine may help protect against novel pandemic influenza A (H1N1), according to the results of a frequency-matched, case-control study in Mexico City reported online October 6 in the British Medical Journal. However, the investigators and authors of an accompanying editorial urge that a specific vaccine against H1N1 flu is still needed.
"The viral genomic sequence for several of the novel A/H1N1 strains, including a Mexican isolate, has been made publicly available," write Lourdes Garcia-Garcia, from Instituto Nacional de Salud Pública in Cuernavaca, Mor, Mexico, and colleagues. "Given the new reassortant nature of this virus — that is, unusual mixing of swine, avian, and human influenza genetic sequences — the available evidence, although incomplete, suggests that seasonal vaccines will confer little or no protection against influenza A/H1N1. Mexican guidelines recommend vaccination with trivalent inactivated influenza vaccine (virus strains A/Brisbane/59/2007 (H1N1)-like, A/Brisbane/10/2007 (H3N2)-like, and B/Florida/4/2006-like antigen) for children aged 6-35 months, all adults aged more than 60, and individuals older than 35 months with conditions conferring a higher risk of influenza related complications."
The goal of this study was to examine the association of 2008–2009 seasonal trivalent inactivated vaccine with cases of influenza A/H1N1 during the epidemic in Mexico. From March to May 2009 at a specialty hospital in Mexico City, 60 patients with laboratory-confirmed influenza A/H1N1 were compared with 180 control patients matched for age and socioeconomic status. Control subjects had diseases other than influenza-like illness or pneumonia and were living in Mexico City or the state of Mexico. The primary study endpoints were odds ratio (OR) and effectiveness of trivalent inactivated vaccine against influenza A/H1N1.
Compared with control patients, patients with laboratory-confirmed influenza A (H1N1) had higher rates of hospitalization, invasive mechanical ventilation, and death. However, control patients were more likely than patients with laboratory-confirmed influenza A (H1N1) to have chronic conditions associated with a higher risk for influenza-related complications. H1N1 influenza was independently associated with receipt of trivalent inactivated vaccine (OR, 0.27; 95% confidence interval [CI], 0.11 – 0.66) and with underlying conditions (OR, 0.15; 95% CI, 0.08 – 0.30), based on the multivariate model. Trivalent inactivated vaccine effectiveness against H1N1 influenza was 73% (95% CI, 34% – 89%), and none of the 8 vaccinated patients with laboratory-confirmed influenza A (H1N1) died.
"Preliminary evidence suggests some protection from the 2008-9 trivalent inactivated vaccine against pandemic influenza A/H1N1 2009, particularly severe forms of the disease, diagnosed in a specialty hospital during the influenza epidemic in Mexico City," the study authors write. "These results are to be considered cautiously and in no way indicate that seasonal vaccine should replace vaccination against pandemic influenza A/H1N1 2009. Our data support the hypothesis that partial protection provided by the seasonal vaccine may be explained by the boosting of existing antibodies that were elicited by previous exposure, through either infection or vaccination, to an influenza A/H1N1 virus genetically and antigenically more closely related to the novel influenza virus than contemporary seasonal H1N1 strains."
Limitations of this study include its retrospective design, small sample size, self-reported vaccine status, and lack of blinding of interviewers to the status of patients with laboratory-confirmed influenza A (H1N1) and control patients.
"The estimates for vaccine effectiveness could be inflated owing to a high prevalence of chronic conditions and vaccination in our control population," the study authors conclude. "Similar studies in other settings are needed to confirm or refute our results."
In an accompanying editorial, Menno D. de Jong, from the Academic Medical Centre of the University of Amsterdam, the Netherlands, and Rogier W. Sanders, from Weill Medical College of Cornell University, New York City, also warn that a specific vaccine against influenza A (H1N1) is still needed. Even in countries that have arranged for vaccine production in sufficient quantities, vaccines may not be available in time.
"Antibodies that recognise multiple influenza strains are rare, which explains the frequent lack of cross protection between vaccines and natural infection," Dr. De Jong and Dr. Sanders write. "However, antibodies have been identified that may open possibilities of developing a 'universal flu vaccine.' Traditional flu vaccines have a good track record in terms of efficacy and safety, so it will take years to replace them, but the new techniques and vaccines at various stages of testing are both necessary and promising."
The Mexican Ministry of Health supported this study. Two of the study authors are employed by Laboratorios de Biológicos y Reactivos de México. Dr. De Jong and Dr. Sanders have disclosed no relevant financial relationships.
BMJ. Published online October 6, 2009.

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