OBJECTIVES

The secondary sex ratio (SSR, i.e., the number of male births per 100 female births) has long been proposed as a sentinel health indicator. Studies have suggested that exogenous environmental stressors reduce SSR after 3 to 6 months (via disproportionate male fetus deaths) and after 9 months (via reduced male conception). We aimed to examine whether SSR declined after the coronavirus disease 2019 (COVID-19) pandemic in Japan.

METHODS

We used monthly vital statistics records collected between January 2013 and April 2021 (the -number of male and female live births, in particular). Using information reported before the pandemic struck Japan (i.e., January 2013 to January 2020), we predicted SSRs for the months after the pandemic (i.e., February 2020 to April 2021) and compared reported and predicted SSRs. We also stratified the analysis by including two groups of prefectures with different degrees of possible influence from the pandemic.

RESULTS

We observed a significant reduction in SSR during December 2020 of 102.81 (i.e., 9-10 months after the pandemic affected the country), which was below the lower bound of the 95% prediction intervals (103.12-106.33). This was the lowest SSR observed during the 100-month study period. In addition, the reduction in SSR during December 2020 was more pronounced in the more seriously affected prefectures.

CONCLUSION

In Japan, we found a significant reduction in SSR (i.e., fewer male live births) 9-10 months after COVID-19 was declared a pandemic. This suggests the onset of the COVID-19 pandemic was a significant population-level stressor.