Victorian researchers have identified 13 cases of measles infection in previously-vaccinated people over the past decade.
The measles vaccine is 96.7% effective when given in one dose (and almost 100% effective in two doses) but, as this research shows, its effects can sometimes wear off over time.
The researchers examined the Victorian health department’s notifiable diseases records for 2008-18 and found 13 patients with PCR-confirmed measles who were IgG-positive but IgM-negative (showing that they had been vaccinated before but had waning immunity).
These patients had a less severe infection than unvaccinated patients and were less likely to report fever, coryza, and cough.
However, one patient with waning immunity did infect two children who were too young for vaccination, showing that these patients could still be contagious.
The number of measles cases ocurring in people with waning immunity increased from zero percent in 2008-13 to 13% in 2014-17, the study showed.
The lack of exposure to the wild measles virus might have contributed to the waning of immunity in recent years, the researchers said.
Australia eliminated measles in 2014, which meant that people weren’t getting a “natural boosting of antibody levels through exposure to circulating wild-type measles”.
Three of the patients with waning immunity in the study had been only given one dose of the measles vaccination, while the rest had two doses or an unknown number of doses.
“It now seems that the immunity that develops after a single dose might not last a lifetime for everyone,” said first author Dr Katherine Gibney, an infectious diseases physician at The Peter Doherty Institute for Infection and Immunity in Melbourne.
The cases were infrequent at the moment “but they will continue to occur and it is important that doctors are aware of this. Making sure everyone, including adults, has two doses of measles vaccine will prevent most of these cases,” she said.
People who had received only one dose of the measles vaccine as children – those born between 1966 to 1994 – were at greater risk of waning immunity than people under the age of 20 who were routinely given two doses as children, Dr Gibney said.
“At this stage there is not much evidence around the potential benefits of a third dose of measles vaccine, although the additional dose appears to be safe,” she said.
Measles is often diagnosed though serology alone, with an IgG-negative/IgM-positive result indicating that the patient has not been vaccinated and has a measles infection.
However, the patients with waning measles immunity appear IgG-positive/IgM-negative, so GPs have to order a PCR RNA test to detect a measles infection.
“The most definitive test is measles virus PCR, which can be done on a throat swab or urine,” Dr Gibney said.
“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,” she said.