Pharmaceutical companies are developing a growing number of vaccine products, according to a study released this week by the Tufts Center for the Study of Drug Development (CSDD).

Researchers found that vaccine products in the R&D pipeline have more than tripled since 2005. Over the same period, worldwide vaccine sales have grown at an average annual rate of 11.5 percent — nearly double the 6 percent growth rate for total pharmaceutical sales.

Annual worldwide sales are on track to reach $40 billion by 2020, according to the report.

“Vaccines have become an important business for many large pharmaceutical companies, and the vaccine market is likely to expand as demand for new vaccines grows,” commented principal investigator Ronald Evens, adjunct research professor at Tufts CSDD and Tufts University School of Medicine.

Growth is driven by a variety of factors, ranging from efforts to reduce health care spending to increasing demand for new vaccines to treat and potentially cure cancer worldwide, Evens said.

The research also found that more than 150 companies worldwide conducted clinical trials for vaccines in 2013, although only 18 firms had products on the U.S. market as of May 2015.

Vaccines accounted for 28 percent of all biotechnology products involved in clinical research in 2013.

Vaccines that are currently in clinical trials offer potential new treatments for more than 40 infectious diseases, including dengue fever, malaria, HIV and Ebola.

Evens highlighted challenges faced by vaccine developers and makers of other pharmaceutical products, such as the need to shorten development times, increase success rates, and contain R&D costs. And he noted that they must also deal with “pockets of public resistance to vaccination in industrialized countries, often based on irrational fears of vaccination side effects.”