A "super-strain" of meningococcal disease, which claimed six lives this year, could become the dominant form in New Zealand if rates follow overseas trends, say scientists.
The Institute of Environmental Science and Research (ESR) has started reporting weekly meningococcal disease group W (MenW) after the number of cases has increased from five in 2016 to 24 this year.
Family physicians and emergency departments have been asked to be vigilant about possible symptoms.
Fatal cases included Whangarei, Alexis Albert, seven years old, whose grieving mother called for a national vaccination and information program, and Dion Hodder, 16, Kerikeri, who died in Auckland City Hospital just after who got sick during an island Motutapu youth camp.
Meningococcal disease is a bacterial infection that causes meningitis – an infection of the membranes covering the brain – and septicemia, or poisoning of the blood.
While meningococcus B (MenB) has long been the dominant strain in New Zealand, causing two-thirds of the disease cases, there are increasing concerns over MenW growth.
The proportion of MenB cases dropped from 67% in 2017 to 49% over the previous year, while MenW rates rose from 11% to 28%.
ESR Public Health Doctor Dr Jill Sherwood said growth in the past two years was "significantly higher" than average annual notifications for 2012-2015, when only 2.5 cases were reported.
The strain was particularly worrying because it was more difficult to diagnose, was more virulent and was involved in higher death rates – possibly up to 15% of cases.
It also affected a larger age range, with more than 40% of cases involving adults over 40 years of age.
New Zealand may follow trends in other countries, such as Australia, where MenW became the dominant strain in 2016.
Although the number of MenW cases in New Zealand is still relatively low, there is an increasing trend in notifications and a recent change in the type of sequence similar to the UK, Australia and Canada.
Whether or when MenW exceeded MenB was difficult to predict because the case numbers were few, Sherwood said.
ESR monitored meningococcal disease by looking for models based on basic demographics such as age, sex, ethnicity, location, living arrangements and onset dates.
"For meningococcal disease, we also analyze the type of strain and group, and how many cases with the same strain and group that were not known to have been in close contact with each other have occurred in certain populations over the past three months."
In the early 2000s, the government launched a national vaccination program following an outbreak of menstruation, but has since been removed from the vaccination program.
However, the Government has now considered public funding vaccinations against new strains of meningococcal disease – including MenW.
Director of the Advisory Center for Immunization, Associate Professor Nikki Turner, said New Zealand had vaccines for some high-risk groups that could be bought but were not in the national program.
"I am aware that our authorities have and continue to think about introducing these vaccines into New Zealand," Turner said.
"It's a little complex and many factors have to be weighed."
Turner noted that vaccines, though effective, did not provide lifelong protection.
Diseases occur mainly in infants, younger and teenage children, and the body is usually transported mostly to teens but not exclusively, "she said.
Plans focusing on these two ages may be most effective. We already have many vaccinations in the birth program, so adding to others can be done, but it's not simple.
Teens are also likely to need a dose and we currently have a vaccinating program visit to middle school but not high school, which may be a better age to go through years of higher risk of adolescents late before vaccination efficacy decreases.
The other complex problem was the role of "immunity to livestock" – where a largely immune population could help protect the few who were not.
If we did a campaign for all young people, we could reduce the body's transport to people's throats and therefore prevent more disease before introducing a vaccine into the program.
New Zealand was not the only country struggling with how to best manage the disease, she said.
"I think there is certainly a role for vaccines, but unfortunately there will not be a magic bullet."
The proportion of group W of all cases of meningococcal disease this year – six of them – 11% in the last year.
49 percent: The share of the dominant B group this year, down from 67% last year.
15%: The potential proportion of MenW cases that are fatal.
40%: At least four out of 10 cases involve patients over 40 years of age.