In this paper, we report the number of tests positive for mumps in South Africa between 2012 to 2017. Most of these cases were reported by the private sector laboratories and occurred mostly in the 1–4 and 5–9 age groups. This age distribution is consistent with what has been reported in other countries during the pre-MuCV era, with most of the infections reported in children below 10 years of age . The cumulative incidence of mumps in our setting was found to be lower than that reported in Western countries during the pre-vaccine era. This most likely reflects under-reporting of mumps since the disease is neither notifiable nor under surveillance. The low incidence could also mean that the reported cases represent patients with more severe presentation of the disease, such as mumps-associated meningitis or orchitis, in whom further investigations would have been conducted. The results could also indicate diagnostic practises in our setting, with mumps possibly only being diagnosed clinically if a patient presented with a typical presentation of parotid enlargement. This would suggest that mumps cases presenting with other complications of the disease could have possibly been undiagnosed and therefore not be accounted for in the analysed results. Also, had there been an outbreak of mumps during the study period, this may have been unidentified.
There have been recent reports on resurgence of mumps infections amongst adolescents and young adults in overcrowded and semi-closed settings such as communes, colleges and camps in developed countries [8,9,10, 14, 25]. In the United States (US), military recruits, a sub-population that has previously been associated with mumps outbreaks, were found not to be involved in the resurgence of mumps infections reported between 1998 and 2007 . This was associated with the decision in 1991, to introduce the MMR vaccine amongst recruits irrespective of previous vaccination status. Although this finding could strengthen a case for booster doses in older age groups, particularly those at high risk such as college students, antibody titres have been found not to be durable, with titres returning to pre-MMR3 dose levels 1 year after vaccination in individuals between 18 and 24 years in a non-outbreak setting . A booster dose of the mumps vaccination is currently recommended only in the setting of an outbreak [9, 10, 26,27,28]. Although the level of protective antibodies and correlates of protection against mumps infection are not well-defined, suggested causes of the resurgence of infections have included waning immunity over time due to a lack of a durable T-cell mediated response, as well as antigenic differences between vaccine and circulating mumps strains, [1, 4, 7,8,9,10, 26,27,28,29,30,31]. As such, the mismatch between vaccine and circulating mumps strains has also prompted the consideration of a polyvalent vaccine [1, 32].
In our study, most of the samples submitted for mumps testing were CSF and blood specimens. One study conducted in Gauteng Province in South Africa used CSF samples from patients who had clinical presentation of central nervous system disease (meningitis, encephalitis or other febrile illness with focal neurological signs) to determine the presence of mumps and to characterise the strains, if found . The study found a low frequency of mumps-associated CNS disease [3/260 (1.2%)], and phylogenetic analysis of one detected strain showed that it was a Jeryl-Lynn or RIT4385 vaccine-like strain. A suggestion made by the authors was the establishing of a mumps surveillance programme in the country, which would also provide valuable mumps epidemiological data. At the time of submitting this paper, there was no established surveillance program for mumps in South Africa.
Our finding of a male predominance with regards to infections is similar to what has been reported in other studies [14, 33]. This has been associated with immunological differences between males and females, where females have been shown to have a stronger T-helper1 cell (Th1) immune response, as well as having persistent and higher antibody levels compared to males [31, 34]. Orchitis has been reported to be the most common complication of mumps infection, and this may also explain the higher proportion of males in this study . Males have also been found to have an increased risk of complications that occur less commonly following mumps infections such as mumps-associated meningitis and encephalitis [6, 34].
The seasonal pattern of mumps infections differs by country, with this difference attributed to environmental, host and viral factors . In our study, we found that the infections peaked in June and November. These months represent the beginning of winter and spring respectively in our setting . A peak in infections in spring and winter has been reported in Jordan .
Although Western Cape was seemingly the most affected province with the highest average yearly incidence, the second highest number of recorded samples was from this province (second to Gauteng). Therefore, this province may have been over-represented in the analysis. The geographic distribution of the infections may also be due to the differential availability of laboratory services in the different provinces in the country.
Formulating recommendations for introducing a MuCV, one of the underutilized vaccines in the African region, in South Africa’s public health sector (through the EPI), is beyond the scope of this paper, and our study results are also not sufficient to inform such a policy . As previously mentioned, before a MuCV can be introduced in a country, the baseline coverage of the measles-containing vaccine (MCV) should be > 80%. Based on the coverage data for the first and second doses of MCV (MCV1 and MCV2 respectively) in South Africa, the MCV1 coverage ranged from 68% in 2007 to 70% in 2018 and was > 80% only during the 2014–2016 period (84% in 2014, 86% in 2015 and 85% in 2016) . The MCV2 vaccine coverage estimates (also from 2007 to 2018) showed a drastic decline, with estimates ranging from 49% in 2007 to 63% in 2016. Of note is that these quoted proportions are WHO and UNICEF estimates, and differ from the country’s official national and administrative estimates, all of which were > 80% for the MCV1 and between 70 and 95% for MCV2 between the 2007–2018 period. Further efforts in increasing the MCV uptake may therefore be required to meet the recommended baseline MCV vaccine coverage before considering the introduction of a MuCV [12, 38, 39]. Another important consideration regarding the introduction of a MuCV in South Africa’s public sector is the vaccine’s schedule compared to that of measles. The first dose of MuCV should be given between 12 and 18 months, and the second dose at the age of school entry (around 6 years of age), whereas, at the time that this paper was written, MCV was being given at 6 (first dose) and 12 months (second dose) according to the EPI schedule [13, 17]. Subsequent to the introduction of the MuCV, determining the effectiveness of the vaccine would be necessary. However, this could be challenged by the lack of knowledge regarding correlates of protection against mumps infection .
The main strength of our study is that we analysed data from both the public and private health sectors. However, our study had several limitations. Firstly, missing data could not be accounted for and information on risk factors was not available since the secondary data that was analysed did not include information about clinical and medical history. Secondly, 50% of mumps infections present non-specifically or with respiratory symptoms, while 20–40% of infections are reportedly asymptomatic or have mild symptoms [1, 27, 40]. These cases may not present at health facilities and would therefore not have been accounted for in the data that we reviewed. Also, data of cases of acute infection where the diagnosis was made clinically without laboratory confirmation would also not be included in our study. Since mumps was not a notifiable disease in South Africa at the time that this paper was written, case-based data that could have supplemented the laboratory-based data were also not available. The above-mentioned limitations may account for the small numbers of mumps test requests, particularly from the public health sector, where mumps infections are likely to be diagnosed clinically rather than by laboratory testing, due to consideration for resources. Thirdly, we were not able to comment on mumps-related complications in our setting because information on clinical presentation or medical history was not included in the analysed data. Fourthly, differential availability of laboratory services across the provinces may also have had an impact on the completeness of the analysed data. The estimates of acute infections presented may be an underestimation of the true burden of mumps disease and may explain why the cumulative incidence found in our study was lower than the cumulative incidence of ≥100 cases/100000 that has been reported in the pre-vaccine era in other settings.