State laws that mandate flu vaccinations for healthcare workers help increase the overall vaccination rate, a new study reports.

The study, led by researchers at the University of Pittsburgh School of Medicine, analyzed immunization records from 2000 to 2011. During that period, the number of states with mandatory vaccination laws for healthcare workers jumped from two to 19.

Between 2000 and 2005, when only Maine and New Hampshire required healthcare workers to get vaccinated, the average immunization rate for those in the healthcare industry was 22.5%. From 2006 to 2011 — a span when 17 other states joined in creating vaccination laws — that rate jumped to 50.9%.

The vaccination laws vary from state to state, with some requiring employers to pay for the vaccines, some mandating workers to have formal documentation, and others making immunizations mandatory for only some healthcare workers — typically long-term care workers — to get vaccinated. The research team gave each state law a ranking based on how broad and strict it is.

“We’re finding that the higher the score — meaning the state has a law and includes components like a mandate or education — the greater the probability that the vaccination rate among healthcare workers will be higher,” said lead researcher Chyongchiou Jen Lin, Ph.D.

Fourteen more states have created mandatory vaccination laws since the study’s completion. Flu vaccination rates among healthcare workers have increased to 66.9%, researchers said.

Results of the study are published in the Journal of the National Medical Association.