• H1N1;
  • influenza;
  • Japan;
  • pandemic;
  • pregnancy


Aim:  No maternal mortality from pandemic (H1N1) 2009 occurred in Japan. However, the reasons for this lack of maternal deaths remain unknown. This study was performed to investigate how many pregnant women were infected, how many women took antiviral drugs for prophylaxis or treatment, and the rate of vaccination effectiveness.

Material and Methods:  A questionnaire study was given to 20 500 postpartum women before leaving obstetric facilities between December 2009 and May 2010 in Hokkaido, asking about antiviral drugs, vaccination, and infection with pandemic (H1N1) 2009.

Results:  Approximately one-third (n = 7535) of women given the questionnaires responded. Of these, 268 women (3.5%) indicated that they had contracted influenza. 353 (4.7%) women took antiviral drugs for prophylaxis after close contact with an infected person and 140 (39.7%) of 353 women finally contracted influenza during or after prophylaxis with antiviral drugs, accounting for 52.2% (140/268) of all patients. 229 (85.4%) of 268 patients took antiviral drug for treatment and 6 (2.2%) needed hospitalization, but not mechanical ventilation or intensive care unit. 196 of 268 (73.1%) patients were already infected before the availability of a vaccine. Among 7328 candidates for vaccination, 4921 (67.2%) were vaccinated. Infection occurred in 0.22% (11/4921) and 2.1% (50/2407) of vaccinated and non-vaccinated women, respectively.

Conclusion:  Frequent use of antiviral drugs for prophylaxis and treatment may partially explain the low infection rate and no maternal mortality from pandemic (H1N1) 2009 in Japan. Vaccination reduced infection by 89% in pregnant Japanese women.