Resurgent global measles: A threat to Australia, New Zealand and Pacific Island Countries

Measles is a highly contagious vaccine-preventable infectious disease that remains an important global health threat. World-wide there has been a resurgence in measles case numbers during 2023, posing an imminent threat to Australasia. While Australia was verified to have eliminated endemic measles in 2014,1 New Zealand in 20172 and Pacific Island Countries are targeting applying for verification in 2025,3 all continue to remain at risk of measles importations from returning residents or overseas travellers.3, 4 This risk of importation rises with increasing global measles case numbers as was dramatically demonstrated during the last global measles resurgence in 2019. In Australia during 2019, measles importations (and secondary import-related cases) mostly by infected returning Australian residents, resulted in just over 280 confirmed cases with over 30 outbreaks across the country.4, 5 Importations came from many countries with large outbreaks at the time, including New Zealand, Thailand, Samoa, Vietnam and the Philippines. Given Australia's high two-dose measles-containing vaccine coverage and population immunity, most outbreaks associated with these importations were small. Alert clinicians who requested appropriate testing were pivotal in prompting rapid public health responses that also helped limit outbreak impact. The situation in New Zealand was more dire with a large outbreak in 2019/2020, with just over 2190 confirmed cases and 774 hospitalisations reported.6 Historic population immunity gaps and persistent immunisation coverage equity gaps, resulted in ongoing chains of measles transmission that persisted for many months, and without the suppression of measles transmission by effective COVID-19 public health measures, New Zealand's measles elimination status was under serious threat.7-9 In a graphic example of the interconnectivity in Australasia, this outbreak spread to Samoa with devastating consequences, with 5707 reported cases and 83 measles-related deaths.10 International progress towards measles eradication was significantly disrupted by the COVID-19 pandemic,11, 12 with global first-dose measles vaccine coverage falling from 86% in 2019 to 83% in 2022.13 The millions of children left vulnerable as a result of this drop in coverage are those now becoming infected with the virus,13 with the epidemiological pattern of case resurgence in 2023/2024 being reminiscent of the early stages of the 2018/2019 measles pandemic (Fig. 1). Given the surge in global measles cases, Australasian clinicians need to be prepared for measles importations and local outbreaks. How can the risk of measles in Australasia be mitigated? Firstly, vaccination, vaccination, vaccination! Australasia's core strategy to reduce the risk posed by measles importations is to raise and then maintain high levels of population immunity through vaccination. The current measles vaccination schedules in Australasia include a first dose of measles-containing vaccine usually at around 12 months followed by a second dose generally in the second year of life. The timing and need for two doses balance the following factors: the risk of infection, the optimal response of the child's immune system to these vaccines and the interference of transplacental maternal measles antibodies.11, 15 The first dose will generally provide excellent protection for most children, but two doses provide even greater protection. At the time of elimination verification in Australia, the estimated efficacy of measles vaccine was 96.7% for one dose and 99.7% for two doses.16 Further strategies to support regional control include considering offering children from 6 months of age who are travelling to countries with endemic measles or current outbreaks, an earlier ‘zero’ dose of measles-mumps-rubella vaccine (MMR0) that may be lifesaving if they encounter measles virus overseas.17, 18 Importantly, an MMR0 dose does not replace the need for two routine vaccines following the usual schedule.17, 18 In addition, strategies to close known immunity gaps should be vigorously pursued. This may entail innovative ways to offer vaccination to age groups or sub-populations that are traditionally harder to reach. Secondly, disease awareness: Think measles! In patients presenting with a fever with or without a rash, it is important to ask about vaccination history, and any recent international travel or contact with people who have travelled internationally. A measles case is infectious 24 h prior to the onset of prodromal symptoms until 4 days after rash onset.11, 19 The maculopapular rash does not appear until 3–7 days into the infection. By the time a rash is identified, the case has potentially exposed a large number of contacts.4 In elimination settings, like Australia, New Zealand and many Pacific nations, it is important to be aware of ‘modified measles’. This presents with a less florid rash, less or absent cough and a more subtle prodrome, and results from a ‘breakthrough infection’.4, 20, 21 Breakthrough measles infections are reasonably uncommon, but can occur due to an inadequate response at the time of vaccination (primary failure) or where initially protective immunity has waned over time (secondary failure).20 Secondary failure may become more common in post-elimination settings where there is absence of regular immune-boosting from circulating wild-strain virus.21, 22 Virus burden and transmissibility appear to be lower in modified measles, but onward transmission may still occur, requiring isolation of cases and public health responses.20, 23 Thirdly, undertake the correct confirmatory tests. Detection of measles IgM antibodies through serological testing is a commonly used diagnostic method but relies upon optimally timed specimens. Approximately 75% of measles cases will have an IgM response 3 days after rash onset, increasing to nearly 100% after 7 days.19 However, an IgM response may not be detected in the setting of breakthrough infection, where polymerase chain reaction (PCR) is much more reliable.23 Given the low burden of measles in post-elimination settings, IgM serology has a low positive predictive value.24 Nucleic acid testing using PCR has greatly improved measles diagnosis. If taken shortly after rash onset, PCR testing of respiratory and/or urine specimens has a high sensitivity and specificity.4, 25 Fourteen days after rash onset, however, it becomes less reliable, but serology remains useful.4, 26 Once received at an accredited laboratory, preliminary PCR results may be available within hours. PCR can also be used to distinguish between wild-type virus and vaccine-strain (genotype A).23 Swabs for PCR are often easier to collect than blood in young children. Swabs of skin lesions are not helpful for diagnosis. Finally, respond quickly! If you suspect a patient has measles, prompt action is critical. Arrange appropriate testing, ensure the patient is isolated at home or under airborne precautions in a health-care facility (until the diagnosis is excluded by laboratory testing or the case is no longer infectious), and contact the local public health unit immediately.4, 19 Following notification of a measles case, a key public health priority is contact management, particularly the identification of susceptible contacts who could benefit from post-exposure prophylaxis with either MMR vaccine or normal human immunoglobulin (NHIG).19 Administration of measles vaccine within 72 h of initial exposure may avert or modify the clinical course of the illness, with effectiveness of post-exposure immunisation ranging from 91% to 100%.23 NHIG can be given within 6 days of initial exposure and it can prevent disease or modify the clinical course of the disease. Where available, NHIG is generally reserved for those for whom the risk of severe disease or measles complications is greatest, including immunocompromised persons, pregnant women and infants too young to be vaccinated.19 Measles is preventable, yet it currently remains one of the most important global public health threats. At least one child in every thousand infections in high-income countries, like Australia and New Zealand, will die usually due to pneumonia or encephalitis,27, 28 and the immunosuppression caused by the measles infection may last weeks to months after infection.11 Preventing every case possible through effective vaccination, maintaining a high degree of clinical suspicion and appropriate testing, and rapidly responding to suspected cases, will continue to save lives. It is also important to redouble global efforts to eliminate measles, with the ultimate goal of eradication, which is biologically and technically feasible.29 Until we achieve this goal, measles anywhere remains a problem everywhere, including in Australasia. Open access publishing facilitated by The University of Newcastle, as part of the Wiley - The University of Newcastle agreement via the Council of Australian University Librarians.