Recently, vaccination with live attenuated influenza virus (LAIV) [identical to FluMist vaccine virus] was reported to enhance subclinical bacterial colonization within the nasopharynx, similar to IAV [influenza A virus]. Although LAIV does not predispose to bacterial pneumonia, whether it may alter bacterial transmigration toward the middle ear, where it could have clinically relevant implications, has not been investigated.
LAIV may increase bacterial transmigration to the middle ear and could thus increase the risk of clinically relevant acute otitis media.
While bacterial AOM [acute otitis media or middle ear infection] often occurs in isolation, increasing evidence suggests that primary or concurrent viral respiratory infections of the URT [upper respiratory tract] may play uniquely important roles in enhancing bacterial acquisition, colonization, and, ultimately, progression from asymptomatic bacterial carriage to AOM notably from Streptococcus pneumoniae and Staphylococcus aureus.
Recently, we demonstrated that LAIV inadvertently enhances the duration and density of bacterial carriage in the nasopharynx of mice, a finding that has since been shown in humans.
A single episode of bacterial middle ear colonization (MEC) was defined as any continuous detection of bacteria that was not interrupted by an interval of >2 days.
Nasopharyngeal carriage of pneumococcus is believed to be a prerequisite for MEC and subsequent pneumococcal AOM, and elevated bacterial density has been associated with transition from asymptomatic carriage to middle ear infections.
Within 12 hours after LAIV inoculation, mice demonstrated an increased incidence of MEC…
By day 4 after LAIV receipt, 85% of mice had at least 1 episode of MEC, compared with only 25% of PBS [phosphate-buffered saline] controls.
[I]nitial onset of MEC in the LAIV group occurred within the first 4 days following vaccination…
By day 10 following LAIV or PBS inoculation, the incidence of MEC in LAIV recipients remained significantly greater than that in PBS controls (85% in LAIV recipients vs 50% in PBS controls.
The duration of MEC was measured for each episode… The duration was significantly increased across all vaccinated groups, regardless of pneumococcal strain… or whether LAIV was given before or following pneumococcal infection.
[B]acteria persisted in the middle ears nearly 2-fold longer [after FluMist vaccination] than in PBS controls.
Live attenuated influenza virus (LAIV) enhances persistence of middle ear colonization (MEC).
The dynamics of increased MEC, with regard to time since vaccination, closely match increased pneumococcal colonizing dynamics of the nasopharynx following WT [wild type] IAV or LAIV virus and support the notion that nasopharyngeal colonizing density may be associated with progression to AOM.