Study Links Live Flu Vaccine With Rise in Pneumococcal Disease

A randomized controlled trial conducted across 10 UK sites found that live attenuated influenza vaccine (LAIV), when given to previously unvaccinated 2-year-old children, transiently increased the density of Streptococcus pneumoniae colonization in those children and raised the odds that their household contacts would go on to acquire the bacterium.¹

The study, published in Lancet Microbe, is the first to demonstrate in humans that LAIV-driven increases in pneumococcal density are associated with increased household transmission of the bacterium.¹

LAIV is offered annually to children aged 2 years and older through the UK's National Health Service childhood immunization program, providing direct protection against influenza for those vaccinated and indirect protection for the wider community. Prior studies in mice, and to a lesser extent in children, had shown that LAIV increases S pneumoniae carriage density, but no study had previously investigated whether that increase translates into greater spread of the bacterium within households.¹

Trial Design and Key Findings

Researchers enrolled 405 families (1379 participants) who had a 2-year-old index child eligible for a first National Health Services-scheduled LAIV dose and at least 2 other household contacts willing to participate. Families were randomly assigned 1:1 to an early LAIV group, in which the index child was immunized at the first study visit, or a late LAIV group, in which the index child served as an unvaccinated control until receiving LAIV 28 days later. Nasopharyngeal swabs were collected from all participants at 5 visits, 2 weeks apart, and tested by quantitative polymerase chain reaction for the pneumococcal autolysin gene, lytA.¹

The trial found that vaccinated index children had 2.5 times higher odds (95% CI, 1.5-4.3; P = .0008) of increased pneumococcal density 14 days after vaccination compared with unvaccinated children. In the same window, household contacts who were pneumococcus-negative at baseline had 2.0 times higher odds (95% CI, 1.2-3.5; P = .015) of acquiring S pneumoniae when their index child had received LAIV, compared with contacts of unvaccinated index children. Both effects attenuated by day 28: the odds ratio for increased density fell to 1.5 (95% CI, 0.88-2.5; P = .14), and the odds ratio for household acquisition fell to 0.89 (95% CI, 0.53-1.5; P = .67).¹

Three serious adverse events were reported during the trial, all related to pregnancy or breastfeeding among household contacts. Investigators determined none were related to LAIV or study procedures. The authors concluded that infection with the attenuated influenza virus increases the risk of pneumococcal acquisition among close contacts of colonized young children in the 2 weeks following vaccination and that this effect appears to be driven by a vaccine-induced rise in bacterial density rather than another mechanism.¹

Study Limitations and Interpretation

The authors note that pneumococcal detection relied solely on the lytA gene target rather than serotype confirmation, which could reduce confidence that a detected acquisition event in a household contact originated from the study's index child specifically. The 2-week interval between sampling visits also limited the precision with which density changes and acquisition events could be tracked.¹

Because the study was a randomized controlled trial, the authors state that concluding a causal relationship between LAIV and the observed excess acquisition among contacts of recipients is reasonable, though they caution that the observed increases in density and transmission were modest and transient relative to the wide range of pneumococcal densities associated with common wild-type respiratory viral infections. The researchers write that these findings raise new questions about the mechanisms by which intranasal LAIV administration might transiently increase infectiousness in colonized children and call for further investigation.¹

Vaccination Timing and Pharmacist Uptake

Pneumococcal disease, including pneumonia, meningitis, bacteremia, sinusitis, and otitis media, remains a substantial contributor to the US health burden, and pharmacists are frequently the point of contact for both administering and counseling on pneumococcal and influenza vaccines.²

The CDC recommends pneumococcal vaccination for children younger than 5 years, adults 50 years or older, and children and adults at increased risk for pneumococcal disease, with 4 vaccines currently available in the United States: PCV21, PCV20, PCV15, and PPSV23. The CDC also advises that conjugate and polysaccharide pneumococcal vaccines should never be administered at the same visit and that, when both are indicated, the conjugate vaccine should be given first.²

Although the UK trial's findings do not change these recommendations, they add texture to the pharmacological interplay pharmacists should understand between live viral vaccines and pneumococcal colonization when counseling families with young children.¹

Separately, a systematic review and meta-analysis of 18 studies examining influenza and pneumococcal vaccine uptake among pharmacists found that pharmacists' own influenza vaccination coverage averaged 50.78% (95% CI, 36.20%-65.22%), while only 1 included study reported pharmacists' pneumococcal vaccination rate, at 20.8%. The review's authors identified doubts about vaccine effectiveness and safety, along with the belief that pharmacists are not at high risk for influenza complications, as recurring barriers to uptake, and recommended continuing education beginning at the pharmacy degree level as a strategy to close the gap.³

REFERENCES

1. Metz J, Qian G, Morales-Aza B, et al. Effects of attenuated influenza virus administration to young children on pneumococcal density and acquisition in UK families: a randomised controlled trial. Lancet Microbe. Published online August 18, 2026. doi:10.1016/j.lanmic.2026.101422

2. Centers for Disease Control and Prevention. Pneumococcal vaccination. CDC. Accessed August 20, 2026. https://www.cdc.gov/vaccines/hcp/by-disease/pneumo.html

3. Waszkiewicz M, Wnuk K, Świtalski J, Augustynowicz A. Influenza and pneumococcal vaccines uptake among pharmacists: systematic review and meta-analysis. Commun Med (Lond). 2025;5(1):258. Published 2025 Jul 1. doi:10.1038/s43856-025-00976-7