The 2019/2020 influenza season in the United States began earlier than any season since the 2009 H1N1 pandemic, with an increase in influenza-like illnesses observed as early as August. Also noteworthy was the numerical domination of influenza B cases early in this influenza season, in contrast to their typically later peak in the past. Here, we dissect the 2019/2020 influenza season not only with regard to its unusually early activity, but also with regard to the relative dynamics of type A and type B cases. We propose that the recent expansion of a novel influenza B/Victoria clade may be associated with this shift in the composition and kinetics of the influenza season in the United States. We use epidemiological transmission models to explore whether changes in the effective reproduction number or short-term cross-immunity between these viruses can explain the dynamics of influenza A and B seasonality. We find support for an increase in the effective reproduction number of influenza B, rather than support for cross-type immunity-driven dynamics. Our findings have clear implications for optimal vaccination strategies.

美国的2019/2020流感季节始于2009年H1N1大流行以来的任何季节，早在8月就观察到类似流感的疾病增加。同样值得注意的是，在本流感季节早期，乙型流感病例在数量上占主导地位，而过去通常是后期高峰。在这里，我们不仅针对2019/2020年流感季节的异常早期活动进行剖析，而且还针对A型和B型病例的相对动态进行剖析。我们建议，新型B / Victoria流行性感冒进化枝的最近扩展可能与美国流感季节的组成和动力学变化有关。我们使用流行病学传播模型来探索这些病毒之间有效繁殖数的变化或短期交叉免疫的变化是否可以解释甲型和乙型流感的季节性变化。我们发现支持增加乙型流感的有效繁殖数量，而不是支持交叉型免疫驱动的动力学。我们的发现对最佳疫苗接种策略具有明显的含义。