H1N1 infection increased risk of premature births for pregnant women with pre-existing medical conditions

Pregnant women with certain medical conditions, such as asthma or heart disease, were at an increased risk of giving birth too early if they were infected with influenza during the H1N1 pandemic, according to a new study by researchers at the Institute for Clinical Evaluative Sciences (ICES) and Children’s Hospital of Eastern Ontario (CHEO) Research Institute.

In particular, when H1N1 was diagnosed in the third trimester of pregnancy, these high-risk pregnant women were three times more likely to have a preterm birth, compared with high-risk pregnant women who did not contract the virus. The study considered women at high risk if they had conditions known to increase the severity of influenza and related complications such as asthma, diabetes, chronic hypertension and heart disease.

The study found the rate of preterm birth in the Ontario population was 6.2 per cent; in the subgroup of women with high-risk conditions, the risk of preterm birth was 9.6 per cent.

However, among healthy pregnant women the study found no link between H1N1 influenza infection and preterm birth.

“Many people were concerned about the health of pregnant women during the 2009 H1N1 pandemic, so it is very reassuring that H1N1 influenza illness was not a major contributor to preterm birth in Ontario’s general obstetrical population,” says Dr. Deshayne Fell, lead author of the study, adjunct scientist at ICES, scientist at the CHEO Research Institute.

“However, going forward, the study suggests that high-risk pregnant women should be considered when developing prevention and treatment plans for seasonal influenza and future influenza pandemics,” adds Dr. Fell, who is also an assistant professor at the University of Ottawa’s School of Epidemiology, Public Health and Preventative Medicine.

The analysis looked at all 192,082 single births  in the Better Outcomes Registry & Network (BORN) Ontario — a provincial registry containing all hospital births greater than 500 grams or 20 weeks of gestation — that were possibly affected by the 2009 H1N1 pandemic by linking the registry with administrative health data. It found that 2,925 pregnant women were diagnosed with pandemic H1N1.

“Our study looked specifically at preterm birth but there may be other complications that can occur when a pregnant woman is exposed to H1N1. Importantly, other studies have shown that vaccinating pregnant women protects the infants from influenza in the early months after they are born, so we always suggest pregnant women get vaccinated,” says study co-author Dr. Jeff Kwong, who is a scientist at ICES and family physician.

The H1N1 pandemic lasted from 2009 to 2010 and was deemed a public health emergency in many countries, including Canada. The most recent estimations suggest that there were between 151,700 and 575,400 deaths worldwide. Since 2009, H1N1 has circulated globally at lower levels.

“The relationship between 2009 pandemic H1N1 influenza during pregnancy and preterm birth: a population-based cohort study,” was published today in the journal Epidemiology.

Author block: Fell DB, Platt RW, Basso O, Wilson K, Kaufman JS, Buckeridge DL, Kwong JC

The Institute for Clinical Evaluative Sciences (ICES) is an independent, non-profit organization that uses population-based health information to produce knowledge on a broad range of healthcare issues. Our unbiased evidence provides measures of health system performance, a clearer understanding of the shifting healthcare needs of Ontarians, and a stimulus for discussion of practical solutions to optimize scarce resources. ICES knowledge is highly regarded in Canada and abroad, and is widely used by government, hospitals, planners, and practitioners to make decisions about care delivery and to develop policy. For the latest ICES news, follow us on Twitter: @ICESOntario

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