Ask people to name Pfizer's best-selling product and many would opt for one of its most famous drugs: Viagra, for erectile dysfunction, or Lipitor, to reduce high cholesterol. But they would all be wrong. The top-seller is not a drug but a vaccine: Prevnar, which prevents pneumonia, meningitis and other infections caused by pneumococcus bacteria. Prevnar generated revenues of $6.25bn last year almost three times as much as Viagra. This was up 40 per cent from the year before, after the expert panel that advises on US vaccine policy recommended its use in over-65s as well as in children. Pfizer is one of just four pharma groups with large vaccines operations. The others are GlaxoSmithKline of the UK, Sanofi of France and Merck of the US. All four reported stronger sales growth in vaccines than in pharmaceuticals last year and operating margins were comparable with pharma at around 25-30 per cent. Pricing remains a sensitive topic, however, especially in the developing world. Médecins Sans Frontières, the health charity, last month launched a challenge against Pfizer's patent on Prevnar in a bid to allow Indian companies to produce the vaccine cheaper. Manica Balasegaram, executive director of the MSF Access Campaign, says it could be produced in India for $6 per child, compared with Pfizer's reduced $10 price. Critics argue that consolidation in the industry has left too few companies, developing too few vaccines " and that those that do exist tend to be aimed at rich countries.