Flu vaccine during pregnancy — is it really safe for the baby?

Aim of the study: Understanding the safety of vaccines is critical to inform decisions about vaccination. Our objective was to conduct a systematic review of the safety of vaccines recommended for children, adults, and pregnant women in the United States.

Results: Of 56,603 reviewed citations, 338 studies reported in 518 publications met inclusion criteria. For children, the strength of evidence (SoE) was high for no increased risk of autism following measles, mumps, and rubella (MMR) vaccine. SoE was high for increased risk of febrile seizures with MMR. There was no evidence of increased risk of intussusception with rotavirus vaccine at the latest follow-up (moderate SoE), nor of diabetes (high SoE). There was no evidence of increased risk or insufficient evidence for key adverse events for newer vaccines such as 9-valent human papillomavirus and meningococcal B vaccines. For adults, there was no evidence of increased risk (varying SoE) or there was insufficient evidence for key adverse events for the new adjuvanted inactivated influenza vaccine and recombinant adjuvanted zoster vaccine. We found no evidence of increased risk (varying SoE) for key adverse events among pregnant women following tetanus, diphtheria, and acellular pertussis vaccine, including stillbirth (moderate SoE).

Aim of the study: To evaluate the effectiveness of influenza vaccination during pregnancy against influenza in infants during their first 6 months of life.

Results: Of the 245,498 infants included in the study, 46.0% were born to vaccinated mothers. The incidence of influenza was lower among infants of vaccinated mothers than among infants of unvaccinated mothers (0.12% vs 0.30%). After adjusting for covariates, vaccination during pregnancy was associated with a 44.4% reduction in infant influenza in any clinical setting (95% CI, 31.4-54.9%). Vaccination during the first trimester was associated with an 11.3% reduction in infant influenza, a 51.5% reduction during the second trimester, and a 59.3% reduction during the third trimester. The differences in vaccine effectiveness estimates comparing the first and second trimesters (P = .02) and the first and third trimesters (P < .001) were statistically significant. Influenza vaccination during pregnancy was associated with a 44.4% reduction in infant influenza infection.

Aim of the study: Infant influenza and pertussis disease causes considerable morbidity and mortality worldwide. We examined the effectiveness of maternal influenza and pertussis vaccines in preventing these diseases in infants.

Results: Among 186 962 pregnant women, 85 830 (45.9%) and 128 060 (68.5%) were vaccinated against influenza and pertussis, respectively. There were 175 and 51 infants with laboratory-confirmed influenza and pertussis disease, respectively. Influenza vaccine effectiveness (VE) was 56.1% (95% CI, 23.3% to 74.9%) for infants aged <2 months and 35.7% (2.2% to 57.7%) for infants aged 2 to <6 months. Pertussis VE was 80.1% (95% CI, 37.1% to 93.7%) for infants aged <2 months and 31.8% (95% CI, -39.1% to 66.6%) for infants aged 2 to <6 months. Our study provides evidence of the direct effectiveness of maternal influenza and pertussis vaccination in preventing these diseases in infants aged <2 months. The findings strengthen the importance of maternal vaccination to prevent these diseases in infants.

Aim of the study: Pregnant women are considered the most important risk group for influenza vaccination. Despite this, the potential risk of harm from the vaccine to the fetus is a key factor in low vaccine uptake. This systematic review aimed to synthesize the best available evidence on the safety of influenza vaccination during pregnancy for fetal development.

Results: A search of the literature was undertaken from the inception of each database up to March 2014. Both observational studies and clinical trials were considered. Fetal outcomes were reported in 19 observational studies, and 14 of those primarily investigated the monovalent influenza A (H1N1) 2009 vaccine. There was significant methodological and clinical heterogeneity among the included studies, so the results were summarized narratively and in tables. Fetal death outcomes for women in later pregnancy ranged from OR 0.34 to 2.95, with 95% confidence intervals crossing or below the null value. Spontaneous abortion before 24 weeks ranged from HR 0.45 to OR 1.23, with 95% confidence intervals crossing or below the null value. Congenital malformations among women vaccinated during their first trimester ranged from OR 0.67 to 2.18, and the imprecise confidence intervals crossed the null value.

Aim of the study: Although international and domestic recommendations advocate influenza vaccination during pregnancy, the vaccination rate among this group in China remains low, mainly because of concerns about fetal safety. This retrospective cohort included mothers who registered and gave birth between August 2020 and July 2023.

Results: The adjusted risk ratios (95% CIs) were: SGA 1.354 (0.842-2.177), LGA 1.504 (0.808-2.798), PTB 0.639 (0.257-1.590), LBW 0.294 (0.072-1.211), low Apgar score 0.918 (0.213-3.958), congenital anomaly 0.676 (0.087-5.247), and NICU admission or referral 0.151 (0.021-1.089). In this study, there was only 1 stillbirth and 1 neonatal death in the unvaccinated group, resulting in a cumulative incidence of 0.1%. No stillbirths or neonatal deaths occurred in the vaccinated group. These findings support public health initiatives aimed at increasing vaccination rates among pregnant women.

Aim of the study: The study aimed to evaluate the safety and impact of maternal influenza, pertussis, and COVID-19 vaccination on perinatal outcomes and identify factors influencing uptake among pregnant women in South Australia.

Results: Overall, 64.1% received influenza vaccine, 74.1% pertussis vaccine, and 29.5% received ≥1 COVID-19 dose during pregnancy. Maternal influenza, pertussis, and COVID-19 vaccination were not associated with increased risks of hypertensive disorders, chorioamnionitis, premature rupture of membranes, antepartum or postpartum haemorrhage, small-for-gestational-age, or low birthweight. Following maternal influenza vaccination, a 13% protective effect against preterm birth was observed, with similar reductions after pertussis vaccination (17% and nearly 50% fewer stillbirths) and COVID-19 vaccination (aHR 0.79, 95% CI 0.68-0.92). Both maternal influenza vaccination (aRR 0.94, 95% CI 0.91-0.98) and pertussis vaccination (aRR 0.89, 95% CI 0.86-0.92) were associated with a lower likelihood of experiencing at least one obstetric complication. A small increase in diagnosed gestational diabetes was observed among women vaccinated against influenza or pertussis.