Most pregnant women who contract bird flu will die, according to an Australian review of infections that found most unborn babies with the virus also die.
Caused by influenza A viruses, a severe strain of bird flu known as highly pathogenic avian influenza A (H5N1) is spreading globally.
While this has caused large outbreaks in poultry and wild birds, spillover infections in mammals, human infections are rare and usually limited to people who work in close contact with sick birds and livestock. There is no evidence of transmission between humans.
There have been increasing numbers of human infections associated with the outbreak in some parts of the world including in China and the US. Most human cases have been mild, with just one severe case in the US.
An infectious diseases researcher with the Murdoch Children’s Research Institute in Melbourne, Dr Rachael Purcell, said while many people who became infected with avian influenza were “completely fine, we wanted to look at what is known about what happens to pregnant women”.
“A pregnant woman’s immune system doesn’t work in the same way as it does prior to pregnancy,” Purcell said. “Unvaccinated pregnant women who get other viruses such as Covid-19 or seasonal influenza often get more sick than non-pregnant women, but we really didn’t know much about what happens to women with avian influenza.”
Purcell and her colleagues examined more than 1,500 research papers to identify any confirmed cases of bird flu in pregnant women. They found 30 such cases across China, Vietnam, Cambodia and the US associated with different strains and outbreaks.
Published in Emerging Infectious Diseases, the review found that 90% of women infected with bird flu during pregnancy died, and almost all of their babies (87%) died with them. Of the babies who survived, most were born prematurely.
“What it highlights to us is that whilst the risk of avian influenza becoming the next human pandemic is thought to be low, it’s really important to think about vulnerable populations and how we might protect them and include them in vaccination programs,” Purcell said.
“Despite being a high-risk population, pregnant women are often excluded from vaccine trials, from priority access to therapeutics, and experience delayed entry into public health vaccination programs.”
There are no specific vaccines for avian influenza in humans, though trial vaccines have been developed for pandemic preparedness in some countries. But these vaccines are not recommended for pregnant women because of a lack of safety data.
“That’s one of the challenges we often have with vaccines, as it is considered unsafe to test them in pregnant women,” Purcell said. “I think as we move forward, what we need to do is think about how we get data on pregnant women.
“If women are enrolled in vaccine safety studies, sometimes those women will inadvertently become pregnant, and there’s an opportunity to ethically study what happens to those women.”
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An infectious diseases specialist at the Australian National University, Associate Prof Sanjaya Senanayake, said while the study sample size was small, it represented the fact that most human cases of avian influenza still relate to direct or close contact with poultry, meaning that pregnant women are less likely to be exposed.
“Of course, if sustained human-to-human transmission occurs with further mutations to the virus, then that will change.”
He added that most of these pregnant women in the study were from developing nations.
“While this is still relevant for a future pandemic of avian influenza, we can’t necessarily generalise such severe outcomes to the developed world with better resourced healthcare settings,” he said.
Despite these limitations, Senanyake said the findings of the study were “likely to be real”.
“We know that pregnant women are more susceptible to serious outcomes with respiratory infections,” he said.
In October the Australian government announced a $95m investment to prepare for avian influenza. Australia is the only continent that remains free of the deadliest bird flu strain.
