Apple Women’s Health Study
The Apple Women’s Health Study is the first long-term research study of this scale and scope that aims to advance the understanding of menstrual cycles and their relationship to various health conditions.
COVID-19 vaccines and menstrual cycles – Data
More details about the information presented in the below tables are included in the study update on COVID-19 vaccines and menstrual cycles.
Table 1: Average change in length of menstrual cycles containing COVID-19 vaccination types compared with pre-vaccination cycles.
| COVID-19 vaccine type | Change in cycle length (95% CI) |
| Cycle with any dose | 0.56 days (0.35, 0.77) |
| First dose mRNA vaccine | 0.50 days (0.22, 0.78) |
| Second dose mRNA vaccine | 0.39 days (0.11, 0.67) |
| Single dose viral vector vaccine | 1.26 days (0.45, 2.07) |
Table 1 represents average change in menstrual cycles during vaccination with all doses of the mRNA vaccines included in the analysis, the Pfizer-BioNTech and Moderna vaccines, or the viral vector vaccine included in this analysis, the Johnson & Johnson/Janssen vaccine. The pre-vaccination range of menstrual cycle variation of + or – 4.2 days was calculated as the standard deviation of menstrual cycle length prior to vaccination in this study, meaning that 70% of participants experience variation in that range during that time.
Table 2: Average change in length of menstrual cycles containing and following COVID-19 vaccination compared with pre-vaccination cycles.
| Cycle containing or following COVID-19 vaccination | Change in cycle length (95% CI) |
| Cycle with any dose | 0.56 days (0.35, 0.77) |
| First cycle after | 0.14 days (-0.13, 0.40) |
| Second cycle after | 0.13 days (-0.14, 0.40) |
| Third cycle after | -0.17 days (-0.43, 0.10) |
| Fourth cycle after | -0.25 days (-0.52, 0.01) |
Table 2 represents average change in menstrual cycles after vaccination with all doses of the mRNA vaccines included in this analysis, the Pfizer-BioNTech and Moderna vaccines, or the viral vector vaccine included in this analysis, the Johnson & Johnson/Janssen vaccine. The pre-vaccination range of menstrual cycle variation of + or – 4.2 days was calculated as the standard deviation of menstrual cycle length prior to vaccination in this study, meaning that 70% of participants experience variation in that range during that time.
In both tables, CI is confidence interval. The first number is the lower limit of the confidence interval. The second number is the upper limit of the confidence interval.
More information is in the full study update: COVID-19 vaccines and menstrual cycles.