Research into measles cases in Victoria has shown some people who were hospitalised with the disease had been vaccinated at least once in the past, but their immunity had waned over time
Published in the Clinical Infectious Diseases journal, the research found that between 2014 and 2017, 13 measles cases involved secondary vaccine failure, meaning the patient had received at least one dose of the vaccine and showed antibodies in their blood, but that their protection waned and they contracted measles.
Ninety per cent of people who are not immune and come into contact with a case will become infected with measles.
The Royal Melbourne Hospital’s Dr Katherine Gibney, an epidemiologist at the Doherty Institute and the Victorian Department of Health and Human Services, said that 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 is not circulating in the community.
Australia was declared measles eliminated by the World Health Organization in 2014; of the 340 measles cases notified in Australia that year, 41% were imported, 34% import-related and the remaining 25% were from unknown sources.
“If you had been vaccinated and came into contact with someone with measles you might get a little natural boost in your antibody levels,” said Dr Gibney.
“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,” Dr Gibney said.
“We do have a definitive test for measles. Our message to doctors is that if you suspect measles, don’t just rely on the serology, which detects antibodies to measles, but also perform a PCR test, which detects the actual virus.”
These results also present a public health problem with transmission, the researchers said; they documented the transmission from one waning immunity case to two infant household contacts, too young for vaccination.
“In most cases, people are unaware they need the second vaccine, or they simply don’t remember if they have had one or two,” Dr Gibney said.
“Anyone who is unsure if they have had two doses of measles vaccine should see their doctor about getting an additional dose. In particular, adults born after 1965 might not have received two doses of measles vaccine during their routine childhood immunisation. The Victorian Government provides the measles vaccine free of charge for these people.
“More work needs to be done in the area of a third vaccine before we can routinely recommend this – we need to know definitively if a third booster shot will extend the immunity to measles for a lifetime.”
The researchers warn that waning immunity cases will likely represent an increasing proportion of notified measles in the years to come.
Men were over-represented in the waning immunity group, they noted – as women might be more likely to receive a second or subsequent dose of measles-containing vaccine in order to protect against rubella if they wish to have a child, or possibly due to differences in health-seeking behaviour.
The researchers also said they hoped that with the recent expansion of the Australian Immunisation Register from a childhood-only to a whole-of-life register, vaccine coverage data across the entire population will gradually improve.
Currently, accredited pharmacists in NSW, the Northern Territory, Queensland, South Australia and Victoria can administer adult MMR boosters, in most cases including to children aged 16 and over.
A recent increase in cases of measles in Australia has seen Federal Health Minister Greg Hunt launch an awareness campaign about the disease and its vaccine.
“I am concerned about the recent increases in measles cases in Australia and want to make sure our community is well protected against this very serious disease,” the Minister said in April 2019.
“As at 5 April 2019, there had been 83 measles notifications in 2019, compared with 103 for the whole of 2018 and 81 for the whole of 2017.”