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Study finds sharp drop in birth rate as a result of miscarriages in 1918 flu pandemic

THE 1918 influenza pandemic in New Zealand resulted in thousands of fewer births, particularly in the 1919 year, largely because of miscarriages from influenza infection during pregnancy, researchers have found.

The study, involving researchers from the University of Otago, Wellington and the University of Auckland, has been published in the New Zealand Medical Journal.

The researchers estimate that in the year 1919 there were 3756 fewer non-Maori and 239 fewer Maori births than in the pre-pandemic year of 1917.

Lead researcher Professor Nick Wilson says the declines represent a “birth rate shock”, as they involve a sharp decline in birth rates per 1000 population of 17 percent for non-Maori and 20 percent for Maori.

The reductions in the birth rate in the pandemic year of 1918 (relative to 1917) were less, at nine percent for non-Maori and seven percent for Maori.

“We estimate that around 80 percent of the birth rate deficit in 1919 was from embryonic and foetal loss due to influenza infection in pregnancy. Some of these would have been miscarriages very early in pregnancy that the pregnant women would not notice — and others which they would have.”

Another of the study authors, Professor Michael Baker, said a smaller role would have been played by the deaths of adults during the pandemic, which could explain about 12 percent of the decline. While this is the first such New Zealand study, the findings are broadly similar to those in other countries, Professor Baker says.

“However, one county in Arizona in the United States had a much higher 43 percent reduction in the birth rate nine to 11 months after the peak of pandemic deaths.”

An immunisation specialist, general practitioner and co-author of the study, Dr Nikki Turner, says the findings of the study can inform better planning around future influenza pandemics.

“In a pandemic, we should prioritise protection of pregnant women from infection using all the proven means available.

“These include actions to avoid contact with sick people, prioritising these women if pandemic vaccines are in short supply, and even prioritising them for access to antivirals and ventilators in hospital intensive care units if they get sick.”

Dr Turner says these issues are also relevant to present-day flu seasons, as seasonal influenza in pregnant women also causes miscarriages.

“There is even an increased risk of death for the women themselves.

“Indeed, the World Health Organisation gives a high priority to vaccinating pregnant women and New Zealand has provided free flu vaccinations for pregnant women since 2010.

“Unfortunately, we don't have good data on the current rate of influenza vaccination in pregnancy in this country, but it is thought to be low, at under half of pregnant women.

Free flu immunisation for pregnant women is available from April 1 each year from a GP surgery or participating pharmacist.

For more information about influenza or the influenza vaccine, talk to your Lead Maternity Carer or doctor, go to www.fightflu.co.nz or call 0800 IMMUNE (0800 466 863).