Sharp drop in birth rate as a result of miscarriages in 1918 flu pandemic, study finds

Professor Nick Wilson.

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, is published in the New Zealand Medical Journal today.

The researchers estimate that in the year 1919 there were 3,756 fewer non-Māori and 239 fewer Māori 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 1,000 population of 17 per cent for non-Māori and 20 per cent for Māori. The reductions in the birth rate in the pandemic year of 1918 (relative to 1917) were less, at nine per cent for non-Māori and seven per cent for Māori.

“We estimate that around 80 per cent of the birth rate deficit in 1919 was from embryonic and fetal 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.”

Professor Michael Baker.

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 per cent 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 per cent 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 Nikki Turner.

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 Organization 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. So, as well as, current efforts to improve childhood immunisation for protection against measles, we also need to improve vaccination in pregnancy against flu.”

• 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 or 0800 466 863 (within New Zealand).

Background facts on the 1918 influenza pandemic

• The pandemic killed an estimated 9,000 New Zealanders (in addition to all the fetal deaths described in this new study).
• It is the largest natural disaster in New Zealand's history – and far greater than the largest non-natural disaster (the plane crash into Mt Erebus, which resulted in 257 deaths).
• It imposed a particularly heavy burden on Māori and on the young adult population (with peak death rates among those aged in their late 20s).
• In response to this pandemic New Zealand invested more in public health and adopted a new public health law in 1920.
• More recently New Zealand has also invested in pandemic plans and stockpiles of some critical supplies.
• In November 2019 Prime Minister Jacinda Ardern unveiled a national memorial to the pandemic.

The research paper, 'One hundred years ago in 1919: New Zealand's birth reduction shock associated with an influenza pandemic' is published in the New Zealand Medical Journal.

For further information contact:

Professor Nick Wilson
Department of Public Health
University of Otago, Wellington
Email nick.wilson@otago.ac.nz

Dr Nikki Turner
Director Immunisation Advisory Centre, University of Auckland and GP, Wellington
Email n.turner@auckland.ac.nz

Professor Michael Baker
Department of Public Health
University of Otago, Wellington
Email michael.baker@otago.ac.nz