One of every four medical schools does not require students to be immunized, contributing to outbreaks of rubella and measles in hospitals and schools, according to a newly reported survey.

Not one medical school surveyed followed all federal vaccination recommendations against preventable, infectious diseases, researchers reported in an article published last week in the Annals of Internal Medicine.

The findings come at a time of concern by U.S. public health officials about a major outbreak of more than 19,000 measles cases so far this year.

Many of the cases were attributed to failure to vaccinate.

Researchers from the Mayo Clinic and the Veterans Affairs Medical Center in Minneapolis surveyed 144 medical schools in the U.S. and Canada; 115 responded. Their report did not name any participating schools.

The failure to require immunization has left students and patients unprotected against preventable diseases, the researchers said.

"When a medical student or doctor gets rubella and sees a pregnant woman or child or elderly patient, the outcome could be deadly," warned the study's lead author, Gregory Poland of the Mayo Clinic.

The survey, completed in 1988, found that 28 percent of the schools responding to the survey had no immunization requirements for entering medical students, who number about 15,000 a year.

Thirty-one percent of the schools had no rubella immunity requirement, 40 percent lacked a measles immunity requirement and 44 percent did not require proof of polio vaccination.

The researchers noted that 97 percent of the schools required at least one immunization before students began their clerkships, a training period in which they help treat patients.

Poland said he was shocked at the report's findings, calling them "unacceptable."

"We're exposed to so much more than the general population," he said, adding that previous studies have found that medical students in clerkships or residency are 13 times more likely to contract measles than the public.

Poland said hospitals and medical schools were "lulled into thinking these diseases weren't a problem anymore," because they saw fewer cases each year.

That lessened the perceived need for vaccinations, he said.

Since the survey was completed, Poland noted, the nation has seen an outbreak of measles and mumps, with cases nearing pre-vaccination levels.

Poland said not one of the schools surveyed followed all the federal Centers for Disease Control recommendations for vaccinations against measles, rubella, mumps, tetanus-diphtheria, hepatitis B and polio.

The CDC also urges medical students to receive annual flu shots.

The authors urged schools to establish immunization policies that include subsidized or free vaccines, methods for monitoring student compliance, and required proof of vaccination from a physician.