Clinical studies sponsored by the pharmaceutical industry are far more likely to find their way into prestigious and high-impact journals than those supported by government or private funding, researchers in Italy have found.

In a systematic review published in the , Tom Jefferson and colleagues looked at 259 ‘primary’ studies (274 datasets) – 48% of which were government-financed – comparing the effects of influenza vaccines with either placebo or no intervention. The mean journal impact factor was calculated for 114 of the studies, as the oldest studies were published in journals for which no impact factor was available, and the mean citation index for 74 studies (similar reasons).

Used as a proxy for journal quality, the impact factor measures the number of citations in the current year to articles published over the previous two years in any given journal, then divides this by the number of substantive articles and reviews published during the previous two years. The citation index refers to the cumulative number of times an article has been cited in journals indexed in the ISI Web of Science database.

For the 92 studies that were government-funded, the mean journal impact factor was 3.74 and the mean citation index was 33.75. For those 52 studies that were wholly or partially funded by industry, the mean impact factor was 8.78 and the mean citation index 59.39.

Among the researchers’ other findings were that higher-quality studies were significantly more likely to show concordance between the data presented and the authors’ conclusions (odds ratio: 16.35), and were less likely to be in favour of a vaccine’s effectiveness (odds ratio: 0.04). Moreover, government-funded studies were less likely to draw conclusions favouring a vaccine (odds ratio: 0.45).

Information overload

Jefferson et al put the issue of study dissemination in the context of an environment where an estimated 7,287 items relevant to primary care are published by journals every month. Even doctors trained in epidemiology would take 627 hours to read and appraise all of these, the authors note. Hence the reliance on browsing abstracts and/or study conclusions, especially in the more prestigious and most readily available journals.

The higher impact factor and citation index for industry-supported studies in the systematic review are “probably a reflection of a higher profile of industry-sponsored studies and a more thorough dissemination of their content”, the researchers suggest.

The pharmaceutical industry is “systematic” in this respect, they comment, adding: “We have personally observed this on three recent occasions in which industry representatives presented abstracts or reprints of industry-sponsored influenza vaccine studies to decision-makers, their advisors, and local researchers in an effort to influence their decisions. Symposiums, conferences, and other types of publication further enhance the dissemination process”.

Moreover, high-impact journals “are preferentially targeted by all studies because of their prominence and prestige, so industry-sponsored studies might have a higher probability of acceptance”, Jefferson et al point out. “The two mechanisms might be linked, but further research, especially in other specialities, is required.”

To improve transparency for readers, though, “we recommend that once a year editors and publishers should post all sources of income related to the running of the journal”, they say.