Flu vaccine during pregnancy — why are critics sounding the alarm?

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, 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 (varied SoE) or 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 (varied 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 unvaccinated mothers (0.12% vs 0.30%). After adjusting for covariates, vaccination during pregnancy was associated with a reduction in infant influenza in any clinical setting by 44.4% (95% CI, 31.4-54.9%). Vaccination during the first trimester was associated with a reduction in infant influenza by 11.3%, a reduction of 51.5% during the second trimester, and a reduction of 59.3% during the third trimester. The differences in vaccine effectiveness estimates that compared the first and second trimesters ( P =.02) and compared the first and third trimesters ( P <.001 were statistically significant. Influenza vaccination during pregnancy was associated with a reduction in infant influenza infection by 44.4%.

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 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 on the fetus is a key factor in low uptake of the vaccine. This systematic review aimed to synthesize the best available evidence on the safety of influenza vaccination during pregnancy on fetal development.

Results: A search of the literature was undertaken from the inception of each database up to March 2014. Both observational and clinical trials were considered. Fetal outcomes were present in 19 observational studies, and 14 of those were primarily investigating the monovalent influenza A (H1N1) 2009 vaccine. There was significant methodological and clinical heterogeneity of the included studies and a narrative summary and tabling of results was performed. 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 less than 24 weeks ranged from HR 0.45 to OR 1.23, with 95% confidence intervals crossing or below the null value. Congenital malformations for women vaccinated during their first trimester ranged from OR 0.67 to 2.18 and imprecise confidence intervals crossed the null value.

Aim of the study: While international and domestic recommendations advocating for influenza vaccination during pregnancy, the vaccination rate among this demographic in China remains low, mainly due to concerns about fetal safety. This retrospective cohort enrolled mothers who registered and delivered 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 endorse public health initiatives aimed at enhancing vaccination rates among pregnant women.

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

Results: Overall, 64.1% received influenza, 74.1% pertussis, 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 (17% and nearly 50% fewer stillbirths) and COVID-19 vaccination (aHR 0.79, 95% CI 0.68-0.92). Both maternal influenza (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.