Almost 50 cases of measles have been confirmed this year, with almost two thirds of the cases linked to the current outbreak in Canterbury.
ESR, the science agency which tracks all notifiable diseases on behalf of the Ministry of Health, says there have been 49 cases to date this year, with outbreaks also in Waikato and the Bay of Plenty.
That compares with seven for the same time last year.
Two years ago, New Zealand won international praise for successfully eliminating endemic measles – meaning the virus was no long circulating in New Zealand.
Prior to that, for three years, the only reported measles cases were imported, or spread from someone bringing the virus into the country.
ESR public health physician Jill Sherwood says outbreaks only occur now when someone with the disease arrives here, and it starts to spread.
"Once here, it can then be easily transmitted if there is a high enough proportion of unimmunised people,” she says.
“That’s why health authorities urge vigilance and stress the need to continue to improve vaccination rates.”
Dr Sherwood says there is always a risk importation of the disease will lead to outbreaks and a return to a situation where measles could become endemic again.
After the introduction of the measles vaccine in 1969, measles continued to occur every year until 1980, with a pattern of “low” years (an average of approximately 100 hospitalisations per year) alternating with “high” or “epidemic” years (an average of 300 hospitalisations per year).
That was because vaccination rates were not high enough to prevent outbreaks.
Increased uptake of the measles vaccine, which is thought to have reached 70 per cent or more by 1980, resulted in this epidemic cycle becoming more accentuated, with fewer cases and longer periods between epidemics.
Dr Sherwood says measles virtually disappeared between the epidemic years which began to occur less frequently – 1984/85, 1991 and 1997. There were 400 hospitalisations in the 1984/85 outbreak, and a total of 943 hospitalisations in the 1991 and 1997 epidemics, with seven deaths in 1991. No deaths occurred in 1997.
She says as vaccination coverage increased, outbreaks have been generally confined to a particular region and have been of shorter duration that the epidemics of the past.
“An outbreak in 2009 was probably stopped and an epidemic prevented by the enhanced immunisation programme that was implemented.”
Dr Sherwood says large-scale measles epidemics occur when the proportion of the population who are non-immune increases, usually because the immunisation coverage is low.
“It’s been estimated that to prevent recurrent outbreaks of measles, 95 per cent of the population must be immune,” she says.
Measles is considered to be the most common vaccine-preventable cause of death among children throughout the world.