Influenza viruses are common causes of acute respiratory illnesses [1]. The recent spread of the H5N1 type in Asia has alerted the public health community to the threat of an influenza pandemic similar to those the world experienced in 1918, 1957 and 1968 [2]. With an estimated 40.3 million people living with HIV infection by the end of 2005[3], it is imperative to evaluate the potential impact of the HIV epidemic on an influenza pandemic and influenza control measures.

Influenza vaccines are the cornerstone of medical interventions aimed at protecting individuals against epidemic influenza. The efficacy of influenza vaccines in HIV infected individuals remains poorly defined. Theoretical safety concerns stem primarily from transient increases in HIV viral loads following influenza vaccination observed in some studies [4–10], although the clinical significance of this phenomenon is unclear. Furthermore, the efficacy of influenza vaccines may be compromised by reduced antibody responses observed in some HIV infected individuals [4, 11–16]. Despite these issues, the US Center for Disease Control and Prevention (CDC) recommends influenza vaccines in HIV-infected individuals [17]. Though this recommendation may be justified by an increased susceptibility to influenza, prolonged viral replication and shedding as well as longer duration of influenza symptomatology and higher influenza-related mortality rates in HIV infected individuals [18], the efficacy and clinical effectiveness of influenza vaccines in HIV-infected individuals is not established. In this paper, the current evidence is reviewed to determine the efficacy and effectiveness of influenza vaccines in preventing influenza illness in HIV infected individuals.

We identified 6 studies that reported the risk of influenza in HIV-infected individuals. However, for this meta- analysis we only included 4 studies, because one study was a duplicate [24] and the other study did not have enough information to calculate an effectiveness measure [25]. In the latter study, 72 HIV-infected individuals were vaccinated with a single dose of 15 μg of the 1998–1999 season influenza vaccine (A/Sydney/5/97 (H3N2), A/Beijing/262/95 (H1N1), B/Beijing/184/93). There were 18 cases (25%) of influenza-like illness characterized by abrupt onset of fever (> 39°C), myalgia and sore throat. Since all of the individuals who were HIV positive were vaccinated, it was impossible to obtain an effectiveness measure and therefore we did not include the study in the meta-analysis.

Table 2 shows the total number of subjects in the vaccine and in the control groups and the number of events in each group for each study. Cases of influenza occurred less frequently in the vaccinated group compared to the non-vaccinated group. All but one study reported a statistically significant protection against influenza by vaccine. The RD ranged from -0.15 (95% confidence interval -0.25 to -0.05) to -0.48 (95% confidence interval -0.63 to -0.34) (Figure 1). In these studies, between 3 and 7 HIV-infected subjects needed to be vaccinated to prevent one case of influenza. However, vaccine effectiveness ranged from 27% to 78% (Table 2). It is worth noting that unlike the RD, VE is a measure of the difference in risks between vaccinated and unvaccinated, accounting for the observed (baseline) risk of disease in people who did not receive vaccine. Thus, despite having the lowest RD, the study conducted in Japan [27] had a relatively high VE of 70%.

Study	Outcome Studied	Vaccine	Control	Risk Difference (95% C.I.)	NNV*	Vaccine effectiveness **
Tasker 1999 [26]	Symptoms of respiratory illness, at least 4 fold increase in antibody titre, viral culture	16/55	23/47	-0.198 (-0.387, -0.01)	5	41%
Fine 2001, [4]	Influenza like illness or a 4 fold increase in antibody titers or isolation of influenza virus	19/42	18/29	-0.168 (-0.404, -0.067)	6	27%
Raineri 2005, [34]	Influenza illness	12/90	34/55	-0.485 (-0.632, -0.337)	3	78%
Yamanaka 2005, [27]	Influenza like symptoms with at least 4 fold increase in antibody titre or viral isolation in culture	16/262	14/66	-0.151 (-0.255, -0.048)	7	71%

C.I.: Confidence Interval; NNV: * Number needed to vaccinate (= 1/RD); ** Vaccine effectiveness = 100*(1-risk ratio)

Ranieri et al [34] only evaluated the effectiveness of vaccines in preventing clinical cases of flu and reported the highest risk reduction (RD of -0.48). The other three studies [4, 24, 26] evaluated both the efficacy of vaccination for prevention of laboratory confirmed (serologically and/or by culture) cases and the effectiveness of vaccines in preventing clinically confirmed cases of influenza (Table 2). Only Tasker et al. reported the number of outcomes separately according to the different case definitions of influenza used in the study [26]. Vaccine significantly reduced the occurrence of influenza for all the three case definitions. The RD was -0.11 (95% confidence interval -0.20 to -0.02) for cases diagnosed using viral culture; -0.20 (95% confidence interval -0.32 to -0.07) for cases diagnosed using a 4-fold increase in antibody titers and -0.26 (95% confidence interval -0.39 to -0.12) for cases diagnosed using either culture or serology. The summary effect measure of effectiveness of the vaccines included clinical and laboratory diagnosis of flu cases. Overall, vaccinating HIV-infected individuals resulted in a significant reduction in the occurrence of influenza cases with a RD of -0.25 (95% confidence interval ranging from -0.43 to -0.08) (Figure 1). This overall estimate should be treated with caution as there was evidence of heterogeneity across studies (chi-square = 14.23, p-value = 0.003).

We assessed publication bias graphically (using a funnel plot) and statistically. The funnel plot displays a plot of the risk difference versus the reciprocal of the standard error. However, because of the small number of studies included in this meta-analysis, it was difficult to visually determine the absence or presence of asymmetry by examining this plot. Nevertheless, both the Begg's rank correlation (p-value = 1.00) and Egger's regression tests (p-value = 0.819) did not show any evidence of publication bias.

This study represents the first published systematic review with meta-analysis of the effectiveness of influenza vaccines in HIV-infected persons. Although we identified a limited number of studies, this review suggests a reduction in the incidence of influenza following vaccination. The magnitude of the reduction is however moderate as evidence by the vaccine effectiveness computed for each of the studies. The vaccine effectiveness was similar to that reported in HIV-negative individuals and in the elderly [28, 29]. Even though influenza vaccine is generally recommended for the latter, in HIV-infected persons, the safety of vaccines in each individual needs to be considered before its administration. In HIV-infected persons, influenza vaccines may result in an increase in HIV plasma level and/or a reduction in CD4 cell counts. These changes, in most cases, have been transient [30], but warrant serious consideration. The evidence of effect heterogeneity in our analysis further supports the need for individual considerations in deciding to administer influenza to each HIV-infected person. Differences in study design, vaccine composition, or study population could be the reason for heterogeneity. However these could not be explored in this analysis because of the limited number of studies identified.

Most previous studies of influenza vaccines in HIV-infected persons focused on their immunogenicity rather than their clinical effectiveness [11–14, 24, 31–33]. These immunogenicity studies confirmed the production of protective antibodies in HIV infected persons even though the antibody levels in HIV infected persons appear to be lower to those in HIV-negative persons. Although the effectiveness of vaccines is mediated by the immune response, the presence of an immune response may not always guarantee clinical effectiveness in preventing infection and/or reducing disease severity following infection. The results of clinical studies reviewed here suggest that immune response levels in some HIV-infected persons may be sufficiently protective against clinical disease. Furthermore, the highest risk reduction reported in Ranieri et al.'s study indicates that vaccine may be more effective in preventing clinical disease than it is in preventing infection per se.

We only identified one randomized study. The three other studies were liable to selection bias and/or confounding. Due to the few number of subjects included in each study, we were not able to assess the effect of the stage of HIV disease (HIV viral load and or CD4 levels) or antiretroviral treatment on the effectiveness of vaccines. In addition, because these studies were conducted in developed countries with predominantly male study populations, their results may not be generalizable to the majority of HIV-infected men and women who live in developing countries. With the threat of a global influenza pandemic, there is an urgent need to evaluate the effectiveness of influenza vaccines in a larger number of representative HIV-infected persons throughout the world.

Current evidence suggests that influenza vaccines are effective, albeit moderately, in reducing the incidence of influenza in HIV-infected individuals. Vaccination is thus a potentially useful intervention and should be considered in the care of HIV-infected individuals. Further studies of clinical effectiveness recruiting larger numbers of HIV-infected subjects and in populations most affected by HIV are, however, required.