How a Flu Vaccine in Pregnancy Helps Protect Babies in Their First Months

What three large studies from Singapore and Australia found about flu hospital stays and baby safety

Babies cannot have their own flu vaccine until they are 6 months old, so the protection their mother passes on during pregnancy is the main shield they have. Three large studies from Singapore and Australia, together covering hundreds of thousands of babies, found that a flu vaccine in pregnancy was linked to fewer babies catching flu or needing hospital care for it, and that it was not linked to any increase in deaths among newborns or infants.

If your baby is in a neonatal intensive care unit (NICU), or you are expecting a baby, you may have been offered a flu vaccine during pregnancy or wondered whether it made a difference. This article explains what the newest research, from Singapore [1] and Australia [2][3], shows in plain terms, and what it means for your family.

Why young babies are so vulnerable to flu

Influenza, usually called flu, is a virus that infects the nose, throat and lungs. For most adults it means a miserable week in bed, but for very young babies it can be much more serious. Researchers who pulled together information from around the world estimated that, in 2018, about 23% of all hospital admissions for flu-related chest infections in children under 5, and about 36% of the deaths in hospital, were in babies younger than 6 months [4]. In the United States, three out of four babies admitted to hospital with flu had been healthy beforehand, so flu is not only a danger for babies who are already unwell [5]. Flu is also easy to miss in babies. A study in four hospitals in Albania, Jordan, Nicaragua and the Philippines found that one in three babies who tested positive for flu was discharged with a diagnosis that did not mention the lungs at all [6].

How doctors came to vaccinate mothers to protect babies

Because flu vaccines are not approved for babies under 6 months, researchers asked whether vaccinating the mother could protect the baby instead [7]. The idea is that a vaccinated mother makes antibodies, the proteins the immune system uses to recognise and fight a germ, and some of these cross the placenta to the baby before birth. A study among Navajo and White Mountain Apache families in the United States confirmed that babies of vaccinated mothers had higher levels of flu antibodies at birth and at 2–3 months old [8].

Some of the strongest evidence came from randomised trials, studies in which a fair draw decides who receives the vaccine, so the groups tend to be alike in every other way. In a trial in Bangladesh, 340 pregnant women received either a flu vaccine or a comparison vaccine; 6 babies in the flu-vaccine group caught confirmed flu, compared with 16 in the comparison group [7]. A larger trial in southern Nepal, where flu circulates all year, involved 3693 women and found that the vaccine reduced flu in babies by about 30% [9]. When that trial was combined with trials in Mali and South Africa, protection was strongest in the first 2 months of life and faded by 4–6 months [10]. Studies in the United States [11] and Canada [12] then showed fewer flu hospital stays among babies of vaccinated mothers in everyday care. Two big questions were left open, however. Most of this everyday evidence came from countries with a single winter flu season, not from tropical places where flu never goes away. And families understandably wanted reassurance, from very large numbers of babies, that the vaccine posed no danger to newborns.

Three studies, three pieces of the puzzle

The three new studies each answered part of these questions, and they fit together. All of them used existing health records, linking information about whether a mother was vaccinated in pregnancy to what later happened to her baby. They were not randomised trials, so the families who chose vaccination may have differed in other ways, and the researchers used statistical methods to make fair comparisons.

The first study, by Sarna and colleagues, asked about safety. It looked at 277,979 babies born from 2015 to 2017 in Queensland, the Northern Territory and Western Australia, using a long-running research project called Links2HealthierBubs, which links the health records of mothers and babies [3][13]. The researchers checked whether babies of vaccinated mothers were more likely to die in their first year.

The second study, by Homaira and colleagues, looked at 147,907 babies of vaccinated mothers in New South Wales, Australia, born from 2016 to 2019. Each was paired with a baby of an unvaccinated mother who was born within 3 days and at a similar stage of pregnancy [2]. The team counted babies who had a laboratory-confirmed flu infection or a hospital stay for flu in their first 6 months.

The third study, by Wee and colleagues, included 221,185 babies born in Singapore between 2017 and 2023 [1]. Singapore is tropical, and flu spreads there all year round. The team counted hospital stays for flu before 6 months of age.

What the studies found

In Singapore, 355 of the 163,800 babies whose mothers were not vaccinated were admitted to hospital with flu before 6 months, about 2 in every 1,000 babies. Among babies whose mothers were vaccinated, the risk was about 41% lower after the researchers accounted for differences between families [1]. Put another way, about 1,100 pregnant women needed to be vaccinated to prevent one baby's flu hospital stay. That number is large because flu admissions are uncommon overall, not because the vaccine works poorly. Importantly, protection was seen whether the mother was vaccinated early, in the middle or late in pregnancy, and it lasted even when the baby's first months fell in the next flu season.

In New South Wales, babies of vaccinated mothers were about 21% less likely to have a recorded flu infection or flu hospital stay in their first 6 months [2]. The researchers called this protection modest but worthwhile, since babies this young have no vaccine of their own. The difference from Singapore is probably explained partly by what was measured: the Australian study counted milder, laboratory-confirmed infections as well as hospital stays, and its records showed only the year a mother was vaccinated, not the date.

In the safety study, 741 of the 277,979 babies died in their first year, about 2.7 in every 1,000. Babies whose mothers had a flu vaccine in pregnancy were not more likely to die at any age in that first year [3]. Fewer of them died in their first week, but this finding became much weaker when the researchers set aside babies who died on the day they were born. The researchers themselves urge caution, because mothers who choose vaccination often have better access to health care. The reassuring message is that the vaccine did not increase deaths; it would be wrong to claim that it prevents them. The same study also looked at the whooping-cough (pertussis) vaccine in pregnancy, which you can read about in our article on protecting babies from whooping cough.

How confident can we be?

Studies based on health records can be affected by differences between families who choose vaccination and those who do not. The Singapore team ran a clever check. If vaccinated families were simply healthier, their babies should also have had fewer hospital stays for illnesses that a flu vaccine should not affect, such as stomach bugs and respiratory syncytial virus (RSV), a common cold-like virus. Those rates were not clearly different, which makes it more likely that the flu protection is real [1]. Vaccination records can also be incomplete. A Western Australian study found that the state's database captured fewer than half of the vaccinations women reported receiving [14]. If gaps like these happen by chance, they usually blur the differences between groups, making a vaccine's effects, good or bad, look smaller than they really are, not bigger. But the gaps were not entirely random: they were largest for women who saw a private obstetrician [14], so it is hard to say exactly how they affected the results.

What this means for your family

If you are pregnant, a flu vaccine is one of the simplest ways to give your baby some protection before they can be vaccinated themselves. The Singapore study suggests it protects babies whenever in pregnancy it is given, so there is no reason to wait for a particular week. It is not a guarantee: some babies of vaccinated mothers still caught flu. For babies born early, the studies included too few babies to say clearly how much protection reaches them, so it is worth asking your care team what this means for your baby.

Many mothers are still not offered the vaccine or are unsure about it. In Singapore, about a quarter of the babies in the study had vaccinated mothers, although the figure was rising [1], and in parts of Australia uptake was about 15% in earlier years [15]. Research across many countries shows that a recommendation from a doctor is one of the strongest reasons women choose to be vaccinated [16]. If you have questions, your midwife, obstetrician or your baby's NICU team are good people to ask.

What researchers are working on next

Researchers want to learn more about protection for babies born early, about whether vaccination early in pregnancy works as well in other tropical countries as it did in Singapore, and about longer-term health beyond the first year. The Australian safety team points out that looking at deaths is only part of the safety picture [3].

The bottom line

Across Singapore and Australia, babies whose mothers had a flu vaccine in pregnancy were less likely to catch flu or be admitted to hospital with it in their first 6 months, and the vaccine was not linked to any increase in deaths among newborns or infants. For parents-to-be, that is reassuring news about a simple, widely available step that helps protect a baby during the months before they can have a flu vaccine of their own.

References

  1. Wee LE, Goh NS, Ho RW, et al. Vertical Vaccination Against Infant Influenza Hospitalization in a Tropical Setting. JAMA Netw Open. 2026;9(8):e2628389. doi:10.1001/jamanetworkopen.2026.28389 ↩
  2. Homaira N, Qian J, Scaria A, Stepien S, Macartney K, Liu B. Effectiveness of Maternal Influenza Vaccination on Influenza and Acute Respiratory Infection in Infants. Pediatr Infect Dis J. 2026;45(6):554-560. doi:10.1097/INF.0000000000005111 ↩
  3. Sarna M, Blyth CC, Moore HC, et al. Neonatal and infant mortality after maternal influenza and pertussis vaccination: Probabilistically linked cohort study. Hum Vaccin Immunother. 2025;21(1):2587307. doi:10.1080/21645515.2025.2587307 ↩
  4. Wang X, Li Y, O'Brien KL, et al. Global burden of respiratory infections associated with seasonal influenza in children under 5 years in 2018: a systematic review and modelling study. Lancet Glob Health. 2020;8(4):e497-e510. doi:10.1016/S2214-109X(19)30545-530545-5) ↩
  5. Chaves SS, Perez A, Farley MM, et al. The burden of influenza hospitalizations in infants from 2003 to 2012, United States. Pediatr Infect Dis J. 2014;33(9):912-919. doi:10.1097/INF.0000000000000321 ↩
  6. Thompson MG, Levine MZ, Bino S, et al. Underdetection of laboratory-confirmed influenza-associated hospital admissions among infants: a multicentre, prospective study. Lancet Child Adolesc Health. 2019;3(11):781-794. doi:10.1016/S2352-4642(19)30246-930246-9) ↩
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  8. Eick AA, Uyeki TM, Klimov A, et al. Maternal influenza vaccination and effect on influenza virus infection in young infants. Arch Pediatr Adolesc Med. 2011;165(2):104-111. doi:10.1001/archpediatrics.2010.192 ↩
  9. Steinhoff MC, Katz J, Englund JA, et al. Year-round influenza immunisation during pregnancy in Nepal: a phase 4, randomised, placebo-controlled trial. Lancet Infect Dis. 2017;17(9):981-989. doi:10.1016/S1473-3099(17)30252-930252-9) ↩
  10. Omer SB, Clark DR, Madhi SA, et al. Efficacy, duration of protection, birth outcomes, and infant growth associated with influenza vaccination in pregnancy: a pooled analysis of three randomised controlled trials. Lancet Respir Med. 2020;8(6):597-608. doi:10.1016/S2213-2600(19)30479-530479-5) ↩
  11. Sahni LC, Olson SM, Halasa NB, et al. Maternal Vaccine Effectiveness Against Influenza-Associated Hospitalizations and Emergency Department Visits in Infants. JAMA Pediatr. 2024;178(2):176-184. doi:10.1001/jamapediatrics.2023.5639 ↩
  12. Fell DB, Russell M, Fung SG, et al. Effectiveness of Influenza Vaccination During Pregnancy Against Laboratory-Confirmed Seasonal Influenza Among Infants Under 6 Months of Age in Ontario, Canada. J Infect Dis. 2024;230(1):e80-e92. doi:10.1093/infdis/jiad539 ↩
  13. Sarna M, Andrews R, Moore H, et al. 'Links2HealthierBubs' cohort study: protocol for a record linkage study on the safety, uptake and effectiveness of influenza and pertussis vaccines among pregnant Australian women. BMJ Open. 2019;9(6):e030277. doi:10.1136/bmjopen-2019-030277 ↩
  14. Regan AK, Mak DB, Moore HC, et al. Surveillance of antenatal influenza vaccination: validity of current systems and recommendations for improvement. BMC Public Health. 2015;15:1155. doi:10.1186/s12889-015-2234-z ↩
  15. Mchugh L, Van Buynder P, Sarna M, et al. Timing and temporal trends of influenza and pertussis vaccinations during pregnancy in three Australian jurisdictions: The Links2HealthierBubs population-based linked cohort study, 2012-2017. Aust N Z J Obstet Gynaecol. 2023;63(1):27-33. doi:10.1111/ajo.13548 ↩
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