Measles virus waning immunity in countries achieving measles elimination

Measles is a highly infectious, vaccine-preventable disease. Yet, in 2017, it caused an estimated 110,000 deaths worldwide, mostly in children under five.

Before the introduction of measles vaccine, major measles epidemics occurred approximately every two to three years and caused an estimated 2.6 million deaths globally each year.

To protect a community, 95 per cent of the population need to receive a measles-containing vaccine. Australia has a high level of vaccine coverage, and in 2014 measles elimination in Australia was declared by the WHO, meaning there is no local transmission of measles, all cases being related to importation of the virus.

Doherty Institute researchers have found the emergence of measles cases in Victoria attributable to waning immunity over the last few years. These individuals with confirmed measles have a prior immunological response to measles virus from either immunisation or natural infection, which can be confirmed by laboratory tests.

Research into measles cases in Victoria between 2014 and 2017 found that 13 people diagnosed with measles and hospitalised were confirmed to have had what is called secondary vaccine failure, meaning they had at least one dose of the vaccine and showed antibodies in their blood, but that protection waned, and they contracted measles.

“Because Australia has done such a good job at eliminating measles, people who have had only one vaccine aren’t getting a natural immunity boost as the disease isn’t circulating in the community,” explains Dr Katherine Gibney, an epidemiologist at the Doherty Institute and the Victorian Department of Health and Human Services.

“Overall, in countries that have eliminated measles transmission, this is likely to emerge as a problem. There isn’t going to be an enormous number of cases, but it will be important in terms of recognising measles, because the cases are a bit different to those who aren’t immune.”

While these cases were hospitalised, the symptoms weren’t considered ‘classic measles’ – patients weren’t reporting fever, cough and runny nose, but they did have a rash.

“Normally, if people have documented receiving two doses of measles vaccine we would be confident they won’t contract measles, but that’s getting greyer – this research has demonstrated some vaccinated people are getting measles,” says Dr Gibney.

These results also present a public health problem with transmission; researchers documented the transmission from one waning immunity case to two infant household contacts, too young for vaccination.

The Doherty Institute is designated a WHO Measles Regional Reference Laboratory for the Western Pacific Region, providing a reference service for measles diagnosis and surveillance for laboratories in the region. It also provides a WHO global proficiency test panel to laboratories worldwide for measles and rubella serology testing.

Ms Suellen Nicholson, Head of Serology for the Royal Melbourne Hospital’s Victorian Infectious Diseases Reference Laboratory (VIDRL) at the Doherty Institute, says that measles reinfection/waning immunity cases have been documented previously in other countries achieving measles elimination.

“Between 2014 and 2018 our laboratory identified six measles waning immunity cases. No waning immunity cases were noted prior to 2014.”