Auteurs : Paul Loubet
CIC Vaccinologie, Cochin, HUPC, AP-HP, Paris
Influenza, or the flu, is a viral infection that affects the respiratory tract. Each year, the flu virus mutates and causes an epidemic that generally lasts from November-December to March-April.
The infection is passed from one person to another either directly, via nasopharyngeal secretions (coughing, sneezing), or indirectly, by contact with the body (hands, mainly) or soiled objects (handkerchiefs, door handles, keys).
Symptoms appear between 1 and 4 days after infection, which generally lasts a week and is characterized by the sudden onset of a high fever, muscle pain, headache, extreme discomfort, a dry cough, a sore throat, and rhinitis. Most people with flu get better in one to two weeks without any specific medical treatment.
Flu poses a greater risk to older people and to patients with certain diseases (heart failure, chronic respiratory failure, diabetes, immunodeficiency), in whom it can cause serious complications. Numerous studies have shown that pregnancy exposes women to greater risk of contracting these “serious” forms of flu, even when they have no comorbidities.
In France, every year the flu affects between 2 and 6 million people, resulting in over 2 million visits to the doctor and 50 000 to 100 000 admissions to the emergency room. The flu causes between 1500 and 2000 deaths a year, essentially among the over-65s. The number of women infected by the flu virus during pregnancy is unknown.
Flu during pregnancy
The risk of hospitalization because of flu complications is 2 to 8 times higher during pregnancy than in women of the same age who are not pregnant. This risk peaks when flu occurs during the third trimester of pregnancy.
Studies conducted during the A/H1N1 influenza pandemic in 2009/2010 reported that between 4 and 13% of deaths occurred in pregnant women. Furthermore, pregnancy multiplied by 4 the risk of admission to intensive care.
What is more, when flu occurs during pregnancy, as with all systemic infections in pregnant women, there is an increased risk of miscarriage or of premature delivery.
Flu vaccination during pregnancy
In France, as in many other countries, vaccination against influenza has, since 2012, been recommended in pregnant women, whatever the trimester of pregnancy.
Vaccination against influenza during pregnancy protects not only the mother, but also her infant, for 6 months after birth, thanks to the mother’s antibodies, which reach the baby through the placenta. In mothers, the flu vaccine reduces by 50 to 70% the risk of infection and by about 40% the risk of hospitalization. Several randomized clinical trials (in Mali, Bangladesh, Nepal, and South Africa) have shown a 35 to 60% reduction in the number of confirmed flu cases in infants born to mothers vaccinated during pregnancy. The trial in Nepal also showed a beneficial effect of vaccination on the birth weight of newborns. Transplacental passage of maternal IgG antibodies is well documented and may protect newborns during their first 6 months of life. This is all the more valuable because infants cannot be vaccinated before 6 months of age.
All relevant studies have shown that vaccination against influenza presents no particular risks in pregnant women. There is no increase in the risk of gestational diabetes, gestational hypertension, or eclampsia in women vaccinated during pregnancy.
Likewise, the risks of miscarriage, prematurity, or congenital anomalies does not differ between vaccinated pregnant women and unvaccinated women. Any early side effects that may arise are the same as those in the general population, that is, pain at the injection site and fever on the day of vaccination.
Influenza vaccine coverage in pregnant women
Despite recommendations and all the data referred to above, unfortunately influenza vaccine uptake among pregnant women is very low. Only 7%, in fact, in France, according to a 2016 national perinatal survey. All health professionals who care for pregnant women have an important role to play in increasing this vaccine coverage. It has been clearly shown that vaccine coverage can be increased by informing pregnant women of the risks of influenza and of the efficacy and safety of the vaccine, and by offering vaccination during pregnancy follow-up appointments.
Vaccination against influenza is recommended during pregnancy, whatever the trimester:
- To protect the mother against potentially serious complications
- To protect the child during the first 6 months of life
In the current state of knowledge, it has never been shown that there is an increased risk of side effects in the mother or unborn child following flu vaccination during pregnancy.
- Haut Conseil de la Santé Publique. Actualisation des recommandations concernant la vaccination antigrippale chez les femmes enceintes. 2012. Disponible sur : http://www.hcsp.fr/explore.cgi/avisrapportsdomaine?clefr=260
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