FIGURE 1.

Flow chart of enrollment and follow‐up of the annual cohorts of China Respiratory Illness Surveillance among Pregnant Women (CRISP), Suzhou, 2015/2016–2017/2018 seasons. After enrolling pregnant women, study nurses conducted twice weekly follow‐up starting at the enrollment date until the delivery date or the end of pregnancy/loss of pregnancy with one phone call and one WeChat text message (a free, instant messaging application widely used in China) to identify episodes of acute respiratory illness (ARI). An ARI episode was defined as ≥1 respiratory symptom (cough, sore throat, stuffy nose, chest pain, and difficulty breathing) and ≥1 systemic symptom (feverish, temperature ≥38°C, chills, and headache) or ≥2 respiratory symptoms