Background:  Virosomal influenza vaccines have been shown to induce better response rates in non-immunocompromized patients. No studies have been performed in HIV⁺ patients, which have directly compared a virosomal influenza vaccine to other formulations.

Methods:  Double-blind, block-randomized trial with a subunit (Influvac 2005/2006) and virosomal vaccine (Influvac plus 2005/2006), manufactured by Solvay Pharma AG (Switzerland). Each vaccine contained 15 μg of hemagglutinin of an A/California/20/99(H3N2)-like, of an A/New Caledonia/20/99 (H1N1)-like, and of a B/Shanghai/361/2002-like strain. Sera were collected at baseline and follow-up for determination of protective anti-HA titres (≥1:40) and seroconversion rates (≥1:4). Side effects were documented with questionnaires. 

Results:  A total of 63 patients received the subunit, and 68 the virosomal vaccine. Baseline characteristics (age, sex, risk for HIV acquisition, and CDC stage) did not differ significantly. Also not significantly different were median absolute (453 vs 398/μL) and relative (23 vs 24%) CD4 cell counts, rates of HAART (76 vs 78%) and of viral loads <50 copies/mL for the subunit and the virosomal vaccine. Both vaccines induced significant anti-HA antibody titres rises in all strains, but these did not differ between vaccines. The A/California strain was associated with the highest titres rises, followed by A/New Caledonia, and were lowest for B/Shanghai. Protective and seroconversion rates behaved similar. Side effects for both vaccines were similar, without significant differences. As there were no statistically significant differences between both vaccines, data were pooled to identify factors for trend in achieving protective anti-HA antibody titres against all vaccine strains. In the multivariate analysis CD4 cell count ≥200//μL (OR 3.3, 95%CI 1.1 to 9.8; p = 0.02) and log viral load (OR 0.8, 95%CI 0.8 to 0.9, p = 0.008) predicted the trend for achieving protective anti-HA titres against all vaccine strains.

Conclusions:  The virosomal vaccine was not associated with higher rates of seroconversion or protective anti-HA antibodies titres in comparison to a subunit vaccine. CD4 cell counts and log of viral load determine the immune response to influenza vaccine.