Conflicting evidence exists on whether changing needleless caps reduces the rate of infections.

Do, A N, et al. “Bloodstream infection associated with needleless device use and the importance of infection-control practices in the home health care setting.” The Journal of infectious diseases 179.2 (1999):442-448.
Cohort study found that rate of bloodstream infections (BSIs) rate “decreased as the frequency of changing the needleless device end cap increased from once weekly up to every 2 days, suggesting that the mechanism for BSI may involve contamination from the end cap.”

McDonald, L C, S N NBanerjee, and W R RJarvis. “Line-associated bloodstream infections in pediatric intensive-care-unit patients associated with a needleless device and intermittent intravenous therapy.” Infection control and hospital epidemiology 19.10 (1998):772-777.
Bloodstream infection (BSI) rates increased in patients receiving “intermittent (vs continuous) intravenous therapy through one or more lumens. The IVAC device valvecomponent was replaced every 6 days, and the endcap used to cover the valve (when connected to an unused lumen) was replaced every 24 hours or after each access. The BSI rate returned to baseline after institution of a policy to replace the entire IVAC device, valve, and endcap every 24 hours.”

Danzig, L E, et al. “Bloodstream infections associated with a needleless intravenous infusion system in patients receiving home infusion therapy.” JAMA: the Journal of the American Medical Association 273.23 (1995):1862-1864.
Researchers conducted case-control and retrospective cohort studies. Data found that needleless device used for total parenteral nutrition and intralipid therapy (TPN/IL) “was associated with increased risk of BSI when injection caps were changed every 7 days.”