HALIFAX -- Four more cases of measles have been confirmed by health officials in the Halifax area, bringing the total number of known cases in Nova Scotia to seven.

Last week, the Nova Scotia Health Authority notified the public about three people who had become infected, saying it was the first time in nine years that the highly contagious infection had been reported.

In an update Thursday, the medical officer of health, Dr. Trevor Arnason, said all of the current cases involve young adults.

Arnason said it's not surprising that more cases have been found given how contagious the virus is.

"At the same time, it's a positive sign that the number remains low and we've had good success following up with contacts of individuals who have contracted measles," Arnason said in a news release.

He said the risk to the general public remained low and that most people are protected by being vaccinated.

The authority said of those people identified, "some were immunized fully or partially and some were not," although no numbers were provided.

It noted people born in the 1970s to early 1990s may have received only one dose of measles-mumps-rubella vaccine in childhood and are eligible for a second dose of the vaccine at no cost through a publicly funded immunization program.

Arnason said that as part of the authority's investigation, some organizations and businesses have been made aware of information they can share with their staff and clients about measles symptoms and about what to do if they develop into an infection.

"It's important to quickly identify those who have symptoms so that precautions can be taken to prevent the spread of measles and follow up with as many people as possible who may have been exposed," he said. "This also gives us an opportunity to look at immunization history and support individuals in getting vaccines up to date."

In an email, the authority said vaccination has been shown to be highly effective, with the efficacy of a single dose given at 12 or 15 months of age estimated to be 85 to 95 per cent. With a second dose, the effectiveness for children approaches 100 per cent.

It said outbreaks do occur in populations with high immunization coverage rates, and at least 95 per cent of the population needs to be immunized to develop herd immunity.

Measles symptoms can include fever, cough, runny nose, red eyes, sleepiness, irritability, small white spots on the inside of the mouth and throat, and a blotchy rash on the face that spreads down the body.