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What Will This Winter’s Flu Season Look Like? The goal of the flu vaccine is to limit the number of severe cases admitted to intensive care and the number of deaths. That is why it is recommended for at-risk individuals and their close contacts. The challenge with this vaccine stems from the frequent mutations of influenza viruses, which require the vaccine to be reformulated every year. Indeed, genetic drift in circulating viruses can occur between the time the strains used to manufacture the flu vaccine are selected and the onset of the seasonal epidemic, leading to reduced vaccine effectiveness.The Morbidity and Mortality Weekly Report (MMWR) from the U.S. Centers for Disease Control and Prevention (CDC) indicated a month ago that recent strains of the A(H3N2) virus differed from the H3N2 Texas vaccine strain included in the 2014–2015 vaccine. In its alert issued in early December (Week 48)(1), it confirmed that among the influenza A(H3N2) strains isolated in the U.S. since October 1, 52% differed from the vaccine strain. Numerous strains of A/Switzerland/9715293/2013 have been found; this virus was selected for the upcoming 2015 vaccine for the Southern Hemisphere (Southern Hemisphere winter during the Northern Hemisphere summer). This strain had already caused a major influenza outbreak in Australia in the New South Wales region (Sydney) last August, with numerous outbreaks in nursing homes.The European Centre for Disease Prevention and Control (ECDC) and the World Health Organization (WHO) Europe recently reported(2) that A(H3N2) strains are predominant in primary care. They belong to sub-groups that are variants of the Texas vaccine strain. This indicates a situation similar to that reported by the CDC in week 48. The A(H1N1) viruses are similar to the California vaccine strain. The ECDC, like the CDC, concludes that, even though vaccine efficacy may be affected, vaccination remains important. Since the H3N2 virus particularly affects the elderly, their vaccination and the implementation of preventive measures in senior care facilities are especially important. However, as influenza activity remains low, it is difficult to be certain about the start date of the epidemic, its severity, and which viruses will be dominant.