Review of the pandemic (H1N1) 2009 among pregnant Japanese women

Pregnant women are prone to serious complications when they contract influenza, and a considerable number of pregnant women died from the pandemic (H1N1) 2009 worldwide; however, no maternal mortality occurred in Japan during this pandemic. This review explores the reasons why maternal mortality did not occur in Japan. Two studies conducted during and soon after the pandemic (H1N1) 2009 in Japan suggested the following: 40,000-50,000 pregnant Japanese women took antiviral medicines for prophylaxis after close contact with an infected person; 40% of them (16,000-20,000) contracted the novel influenza and accounted for a half of all 30,000-40,000 pregnant patients with the novel influenza; at least 181 of them required hospitalization; and at least 17 of them developed pneumonitis. Hospitalized women had a 2.5 times higher risk of preterm delivery (at <37 weeks) compared with the general population. The two studies suggested that the following may have contributed to the lack of maternal mortality in Japan: (i) more than 60% of candidates were vaccinated within 1.5 months after the availability of a vaccine against the novel virus; (ii) vaccination reduced the infection rate by 89%; (iii) a large number of women took antiviral drugs before symptom onset after close contact with an infected person; and (iv) approximately 90% of hospitalized pregnant patients took antiviral drugs within 48 hours after symptom onset.