The biological differences associated with the sex of an individual are a major source of variation, affecting immune responses to vaccination. Compelling clinical data illustrate that men and women differ in their innate, humoral, and cell-mediated responses to viral vaccines. Sex affects the frequency and severity of adverse effects of vaccination, including fever, pain, and inflammation. Pregnancy can also substantially alter immune responses to vaccines. Data from clinical trials and animal models of vaccine efficacy lay the groundwork for future studies aimed at identifying the biological mechanisms that underlie sex-specific responses to vaccines, including genetic and hormonal factors. An understanding and appreciation of the effect of sex and pregnancy on immune responses might change the strategies used by public health officials to start efficient vaccination programmes (optimising the timing and dose of the vaccine so that the maximum number of people are immunised), ensure sufficient levels of immune responses, minimise adverse effects, and allow for more efficient protection of populations that are high priority (eg, pregnant women and individuals with comorbid conditions).
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