Alberta officially declared a measles outbreak in Calgary, Edmonton and central Alberta on Tuesday.

Health officials say the declaration was prompted by "several consecutive weeks in which new cases of measles disease have been confirmed."

Dr. Marcia Johnson, medical officer of health in the Edmonton Zone for Alberta Health Services, declared a measles outbreak Tuesday. ( Gareth Hampshire/CBC)

Dr. Marcia Johnson, medical officer of health for the Edmonton Zone, said there have been 22 cases of measles in the province, but because the disease has a long incubation period, more cases are expected to emerge.

The declaration means changes to the recommendations for routine immunization for children, she said.

"It allows us to offer measles vaccine early to help protect infants over the age of six months to just under 12 months. Those infants usually are not eligible for measles vaccine and therefore are not protected against the disease."

Alberta Health Services recommends:

Infants six months of age to under 12 months of age are now eligible for an early additional dose of measles vaccine for short-term added protection against the disease. That includes babies living in the zones or travelling to those zones, or any out-of-province destinations with measles outbreaks. Those children will still need an additional dose of the vaccine once they are four.

Children age four and older who are attending daycares, day homes and schools within the regions and have not yet received their second dose of measles vaccine should receive it as soon as possible.

The outbreak applies to the Calgary Zone (nine measles cases), the Edmonton Zone (six cases) and the Central Alberta Zone (seven cases), but not to any other part of the province.

Health officials are urging all Albertans born in or after 1970 to make sure they have received both doses of the measles vaccine.