Influenza Vaccine Triggers Uneven Antibody Responses in Adults Living With HIV
A single dose of trivalent inactivated influenza vaccine generated substantial antibody responses in Chinese adults living with HIV, but the strength of that response depended heavily on the strain. Protection against A(H1N1) looked robust, while responses to A(H3N2) and B/Victoria lagged behind those seen in HIV-uninfected adults. The pattern fits a familiar immunologic reality: HIV can blunt vaccine responsiveness even in the era of antiretroviral therapy, and strain-specific immunity rarely moves in lockstep across influenza antigens.
Investigators conducted a single-center, open-label comparative analysis in Hangzhou, enrolling vaccinated adults living with HIV alongside age- and sex-comparable HIV-uninfected adults. A planned 472 vaccinated participants were expected, and 489 were ultimately enrolled, including 237 adults living with HIV and 252 HIV-uninfected adults. All participants received one 0.5 mL intramuscular dose on Day 0, with blood drawn before vaccination and again 28 days later for hemagglutination inhibition testing against A(H1N1), A(H3N2), and B/Victoria.
One result stands out immediately. A(H1N1) seroconversion was high in both groups, but the other two strains exposed a clear immunogenicity gap.
Strain-Specific Differences Shaped the Immunogenicity Readout
At 28 days, seroconversion for A(H1N1) reached 91.00% in adults living with HIV and 86.69% in HIV-uninfected adults, a difference that was not statistically significant. The picture changed sharply for A(H3N2), where seroconversion was 49.50% in the HIV group versus 89.52% in the comparison group, and for B/Victoria, where rates were 40.00% and 64.52%, respectively. Both of those differences were statistically significant. Seroprotection followed the same direction: 94.00% versus 97.98% for A(H1N1), 63.50% versus 97.18% for A(H3N2), and 40.50% versus 68.55% for B/Victoria.
The geometric mean titers were also lower in adults living with HIV for all three strains. Geometric mean increase moved in opposite directions depending on the antigen: higher for A(H1N1), lower for A(H3N2) and B/Victoria. In practical immunologic terms, that means the vaccine could still produce a measurable rise in antibody concentration, but the rise was not equally durable or efficient across all viral targets. Influenza vaccines rely on antigen presentation, B-cell activation, and memory responses; chronic HIV infection can leave those pathways partially impaired even when patients are clinically stable.
HIV Infection Remained an Independent Predictor After Adjustment
Multivariable analysis did not wash away the association between HIV infection and weaker vaccine responses. After adjustment for demographic and clinical factors, HIV status remained independently linked to several post-vaccination immune outcomes, particularly for A(H3N2). Baseline HI antibody titer also emerged as a consistent determinant of post-vaccination response, a finding that is biologically plausible because pre-existing antibody levels can shape the magnitude of fold-rise after antigen exposure.
For clinicians, the signal is straightforward: adult HIV status alone should prompt expectations of somewhat reduced influenza vaccine immunogenicity, even before CD4 count, ART duration, or other covariates enter the conversation. The study also explored recent CD4+ T-cell count, time since HIV diagnosis, and duration of antiretroviral therapy, although the main message remained anchored in the strain-specific gap between groups rather than any single laboratory marker.
Safety Profile Remained Reassuring Across Six Months
Safety data were clean. Most adverse events were mild, and no Grade 4 reactions or vaccine-related serious adverse events were reported during 6 months of follow-up. That matters because vaccine hesitancy in immunocompromised populations often grows from concern about tolerability rather than concern about efficacy, yet this cohort did not show a meaningful safety penalty after vaccination.
Solicited and unsolicited adverse events were monitored after immunization, while serious events were followed throughout the study period. The absence of vaccine-related serious harm supports routine use of inactivated influenza vaccination in adults living with HIV, especially when the alternative is a seasonal infection that can carry more severe consequences in this population because of persistent immune dysregulation and a higher burden of comorbidity.
Descriptive Influenza-Like Illness Data Did Not Test Vaccine Efficacy
The parent trial also included an unvaccinated cohort of adults living with HIV, but those data served only a descriptive role. Few influenza-like illness events occurred between 29 days and 6 months after vaccination among both vaccinated and unvaccinated participants living with HIV, yet those cases were not virologically confirmed and were not used to assess vaccine efficacy.
That distinction is essential. Influenza-like illness can be caused by many respiratory pathogens, so symptom-based surveillance alone cannot substitute for confirmed influenza endpoints. The investigators appropriately framed this material as exploratory and hypothesis-generating rather than a measure of clinical protection.
Why A(H3N2) and B/Victoria May Be Harder to Tame
The imbalance across strains is not surprising from an immunologic standpoint. A(H3N2) has long been one of the more difficult influenza targets for vaccine-induced protection, partly because of antigenic drift and partly because antibody responses to this subtype can be less durable or less potent in vulnerable hosts. B/Victoria can pose a similar challenge when immune memory is incomplete or the host response is constrained by chronic infection. In adults living with HIV, altered B-cell function, residual immune activation, and incomplete restoration of immune architecture may all contribute to the weaker fold-rise seen here.
The strongest signal in this study is not that vaccination failed. It did not. The stronger message is that influenza vaccination in adults living with HIV produced meaningful responses, but not uniformly across all strains, and not to the same degree achieved in HIV-uninfected adults.
What This Means for Influenza Prevention in HIV Care
These findings support continued influenza vaccination for adults living with HIV in China and reinforce the public health logic behind annual immunization in immunocompromised populations. They also suggest that post-vaccination protection may not be equivalent across influenza strains, a point with real bedside consequences during seasons dominated by A(H3N2) or B-lineage circulation. For primary care and infectious diseases teams, the study argues for routine vaccination rather than therapeutic nihilism, while also leaving open the question of whether alternative formulations, booster strategies, or different timing relative to immune status could improve responses further.
The trial was registered in China Clinical Trial Registry under ChiCTR2500112191. Its central limitation is built into the design: immunogenicity was measured, safety was measured, and influenza-like illness was only explored descriptively. Clinical efficacy against laboratory-confirmed influenza was not tested, so the next step has to be a study that can link these antibody readouts to actual seasonal protection in adults living with HIV.