Data from: Human birth seasonality: latitudinal gradient and interplay with childhood disease dynamics

More than a century of ecological studies have demonstrated the importance of demography in shaping spatial and temporal variation in population dynamics. Surprisingly, the impact of seasonal recruitment on infectious disease systems has received much less attention. Here, we present data encompassing 78 years of monthly natality in the USA, and reveal pronounced seasonality in birth rates, with geographical and temporal variation in both the peak birth timing and amplitude. The timing of annual birth pulses followed a latitudinal gradient, with northern states exhibiting spring/summer peaks and southern states exhibiting autumn peaks, a pattern we also observed throughout the Northern Hemisphere. Additionally, the amplitude of United States birth seasonality was more than twofold greater in southern states versus those in the north. Next, we examined the dynamical impact of birth seasonality on childhood disease incidence, using a mechanistic model of measles. Birth seasonality was found to have the potential to alter the magnitude and periodicity of epidemics, with the effect dependent on both birth peak timing and amplitude. In a simulation study, we fitted an susceptible-exposed-infected-recovered model to simulated data, and demonstrated that ignoring birth seasonality can bias the estimation of critical epidemiological parameters. Finally, we carried out statistical inference using historical measles incidence data from New York City. Our analyses did not identify the predicted systematic biases in parameter estimates. This may be owing to the well-known frequency-locking between measles epidemics and seasonal transmission rates, or may arise from substantial uncertainty in multiple model parameters and estimation stochasticity.

一个多世纪以来的生态学研究已证实，种群统计学（demography）在塑造种群动态的时空变异中具有核心作用。令人意外的是，季节性种群补充（seasonal recruitment）对传染病系统的影响却未受到足够重视。
本文呈现了一套涵盖美国78年月度出生率（natality）的数据集，据此揭示出出生率存在显著的季节节律：出生率峰值的出现时机与波动幅度均存在地理与时间维度上的变异。年度出生脉冲（birth pulses）的发生时序呈现纬度梯度（latitudinal gradient）特征——北部各州的出生率峰值集中于春夏季，南部各州则以秋季为峰值期，这一模式在整个北半球均有观测记录。
此外，美国出生季节节律的波动幅度在南部各州较北部各州高出两倍以上。随后，我们借助麻疹（measles）的机理模型，探究了出生季节节律对儿童传染病发病率的动态影响。研究发现，出生季节节律可改变传染病流行的规模与周期，其效应取决于出生峰值的出现时机与波动幅度。
在一项模拟研究中，我们将易感-暴露-感染-恢复（susceptible-exposed-infected-recovered, SEIR）模型拟合至模拟数据，结果表明，忽略出生季节节律会对关键流行病学参数（epidemiological parameters）的估算产生偏倚。最后，我们使用纽约市的历史麻疹发病率数据开展统计推断。分析结果并未发现参数估算中存在预期的系统性偏倚。这一现象可能源于麻疹流行与季节性传播速率之间众所周知的频率锁定（frequency-locking）效应，亦或是源于多个模型参数的显著不确定性与估算过程中的随机性。