Children of mothers who had the 2009 pandemic H1N1 “swine flu” vaccination during pregnancy are no more likely than other children to have health problems during their first five years of life, research* published today in The BMJ has concluded. Experts commented that the “resounding lack of evidence of harm from flu vaccination in pregnancy” means that, especially in the era of “anti-vaxx” anxiety and misinformation, it is health professionals’ duty to speak out loudly and clearly: vaccination of pregnant women saves lives.
A research team from Canada and the US explained that for many years official policies – in Canada, the US and elsewhere – have advised all pregnant women to be immunised against influenza, because of their high risk of complications from flu. In addition, vaccine-derived maternal antibodies cross the placenta and confer passive immunity to infants during the first months of life. Despite this, concern about safety is commonly cited as a reason for low uptake of influenza vaccine among pregnant women.
The researchers linked a population-based birth registry with health administrative databases in the province of Ontario, covering all 104,249 live births from November 2009 to October 2010; they had follow-up information on the children’s health up to five years old.
They reported that of these births, 31,295 (30%) were exposed to pH1N1 influenza vaccination in utero. They found no significant associations with upper or lower respiratory infections, otitis media, any infectious diseases, neoplasms, sensory disorders, urgent and inpatient health services use, paediatric complex chronic conditions, or mortality. There was a weak association, but just significant, between prenatal pH1N1 vaccination and increased risk of asthma (adjusted hazard ratio 1.05) and decreased rates of gastrointestinal infections (adjusted incidence rate ratio 0.94).
These results were unchanged in sensitivity analyses accounting for any potential differential healthcare seeking behaviour or access between exposure groups.
The study authors commented: “Overall, our findings indicate that 2009 pH1N1 influenza vaccination in pregnancy was not associated with negative five-year health outcomes in children, which is reassuring and consistent with a similar recent study from Denmark. Although we observed a small, but statistically significant, increase in paediatric asthma and a reduction in gastrointestinal infections, we are not aware of any biologic mechanisms to explain these findings.
“Future studies in different settings and with different influenza vaccine formulations will be important for developing the evidence base on longer-term paediatric outcomes following influenza vaccination during pregnancy.”
In an accompanying editorial**, two public health specialists said there is a clear message that influenza vaccination during pregnancy is, by all available evidence, safe for mother and offspring. They argued that this accumulating evidence requires two responses from the medical community: as researchers, to question the evidence’s strengths and probe its weakness, then weigh its contribution; and as doctors, to disseminate that evidence. They said: “As physicians entrusted with the health of individuals and populations… when the facts allow, we are charged with speaking out loudly and clearly.
“In this case, vaccination of pregnant women averts a small but serious risk of death and morbidity for the mother and a chance of death of the foetus. A fear of harm to the child is ungrounded – children have no remotely comparable risk. Especially in this era of ‘anti-vaxx’ anxiety and misinformation, it is our duty to be clear: vaccination of pregnant women saves lives.”
*Walsh LK, Donelle J, Dodds L, et al. Health outcomes of young children born to mothers who received 2009 pandemic H1N1 influenza vaccination during pregnancy: retrospective cohort study. BMJ 2019; 366:l4151 doi: 10.1136/bmj.l4151
**Håberg SE, Wilcox AJ. Editorial: Flu vaccination in pregnancy. BMJ 2019; 366:l4454 doi: 10.1136/bmj.l4454