Monitoring the Effect of Vaccination on Mumps Cases Complications in the Czech Republic - Surveillance Data 2013–2022

Mumps is a viral respiratory infection caused by an RNA virus of the family Paramyxoviridae, genus Rubulavirus (1). The disease starts with nonspecific prodromal symptoms: malaise, fever, myalgia, headache and anorexia (1). The common course of the disease is acute parotitis in up to 70% of cases. The following complications may occur: orchitis (the most frequent complication), oophoritis, meningitis, encephalitis, pancreatitis, myocarditis, deafness and so forth (1). Asymptomatic and subclinical mumps infections can account for up to 20% of all infections (2). The incubation period is 16–19 days (range 12–25 days) (1). Mumps is transmitted by droplet spread and by direct contact with the saliva of an infected person (1).

It belongs to notifiable diseases only in some countries. The global annual number of reported cases in the years 1999–2022 ranged from 169,799 in 2019 to a maximum of 726,638 in 2011, based on the World Health Organization (WHO) Global Health Observatory data repository (3).

The incidence of mumps per 100,000 population reported in the European Union/European Economic Area (EU/EEA) in 2013–2021 decreased from 5.9 in 2013 to 0.4 cases in 2021. The most recent EU/EEA data available before the COVID-19 pandemic in 2018 show an incidence of 2.6 cases/100,000 population, and a higher proportion of cases occurred in males in all age groups. Hospitalisations and complications due to mumps in 2018 were rare (4.5% and 6.5%, respectively), with no deaths reported. The Czech Republic (CZ) belonged to those EU/EEA countries with the highest notification rate in 2018 (5.1 cases/100,000 population). In 2021, 1,567 cases of mumps were reported by 27 EU/EEA Member States, with an overall notification rate of 0.4 cases/100,000 population. This was significantly lower than the notification rates reported during the previous four years (range 1.7–4.2). In 2022, 27 EU/EEA countries reported 2,593 mumps cases, with an overall notification rate of 0.7 cases/100,000 population (4).

This study builds on the work of Czech authors confirming the protective effect of MMR vaccination against mumps complications for data covering the period 2007–2012 (12). In our study, teenagers were the most affected age group. This result is in line with studies which have demonstrated that the highest burden of disease is currently among adolescents and young adults (12,13,14,15).

Due to the COVID-19 pandemic, it could not be ruled out that some mild mumps cases have not been detected in CZ. Mumps is a respiratory disease, so the implementation of non-pharmaceutical interventions (NPIs) during the COVID-19 pandemic may have influenced and reduced the number of mumps cases reported in CZ; a similar decrease of mumps cases was reported in other countries (4).

Several studies confirmed the reduction in the incidence of many communicable diseases, including several vaccine-preventable diseases during the pandemic, most likely due to social distancing, school closures and other NPIs reducing the likelihood of transmission (16,17,18).

In the last decade, mumps outbreaks were reported in several regions of CZ, with the highest incidence in South Bohemia in 2016 (19). The increase in mumps incidence in the highly vaccinated population in CZ seems to be a consequence of a secondary vaccination failure due to waning immunity over time (20, 21). This view is supported by the fact that in the mumps outbreak in South Bohemia, the age group 15–19 years was the most affected (19,20,21). The results of the serological surveys from 2013 demonstrated that only 33% of individuals in the age group 15–19 years had mumps antibodies (19, 22). Genotype G, which is predominant in Europe, has also been detected in CZ (20).

The most frequent complication in our study was mumps orchitis, which occurred in 9% of male cases, while in the previous CZ study it was reported in 12% of males. In the current study, a smaller proportion of cases were hospitalised due to the clinical course of the disease (8%) than in the previous CZ study with 12% hospitalised cases (12). We hypothesized that the higher number of hospitalisations in males could be due to the number of reported orchitis cases, which reveals a more severe course of mumps. In a Spanish study, the most frequent complication among hospitalised mumps cases was meningitis (13).

In our study, 19% of the unvaccinated participants had complications, while among one- or two-dose vaccine recipients the rate was about 5%. The reason for that difference is in vaccination status itself.

Another large retrospective cohort study analysing Korean mumps data reported that only 2% of patients had related complications (less than in our study) and the most reported complication was mumps orchitis among 42% of the males with complications, while meningitis was most frequent among females. The Korean authors concluded that in their study mumps complications were more frequent among males than in females (15) which is in line with our study results, where males accounted for 88% of all the complications.

We found that two doses of vaccine significantly reduced the risk of hospitalisation, corresponding to aVE 57% and the risk of any complication corresponding to aVE 52% in the study period 2013–2022, while in the previous CZ study the aVE was higher (71% and 68%, respectively) (12). In a large mumps outbreak in the Jerusalem district, authors estimated aVE against complications as 63% and aVE against hospitalisation as 44%, both for two doses of vaccine (23).

Our study results also demonstrated that two doses of vaccine significantly reduced the risk of orchitis and meningitis corresponding to aVE 50% and 59%, respectively. The previous CZ study showed aVE against orchitis of 72% and against meningitis aVE of 64% (12). We used the same methodology as in the previous CZ study, therefore we assume that probably other factors could play a role resulting in a lower aVE in our study, as the proportion of complications was similar in both CZ studies (e.g. the vaccination status by itself, worse quality of the source data inserted in the notification system). The rate of unknown vaccine status in the two CZ studies (current one and in the years 2007–2012) is not similar. In the current CZ study 4% of mumps cases had unknown vaccination status. In addition, another 4% of cases in the current study had an unknown number of vaccine doses. In a previous CZ study (2007–2012) vaccination status of mumps cases was: 0 doses (17% of cases), 1 dose (<1% of cases), 2 doses (82% of cases) and 3 doses (<1% of cases).

Although mumps incidence was the highest among teenagers, the highest proportion of mumps complications and hospitalisations was reported in the 35–44 years age group. In the Spanish study, hospitalisation and complication rates increased over time in the age group 25–34 years (13).

Among the two-dose recipients, the rate of complications increased from 0 to 12% in the post-vaccination interval categories after the second dose from 0–4 years to 20–24 years, then decreased slightly to 9% at 25+ years. These results of the effect of a time interval after the second dose of the vaccine on the risk of mumps complications are in line with the previous CZ study, when the rate of complications was higher and increased up to 16% (12). The childhood immunisation programme against MMR in CZ has changed since January 2018, and not enough data for the evaluation of the new schedule was available. As of 31 December 2020, more than 95% of those born in 2017 received at least one dose of the MMR vaccine in different regions. It is important to improve VC with two doses of MMR vaccine, especially in regions with lower VC. The administrative control of vaccination in CZ was cancelled from 1 January 2022 and its results should be replaced by VC estimates obtaining data from health insurance companies (8, 24).

As part of ongoing surveillance, it is necessary to maintain high data quality when entering it in the notification system.

Reporting bias may have been present in our study, especially for the vaccination status variable, which could not be verified as the national vaccination register was launched only in January 2023. In addition, the legislation valid until the end of 2017 did not explicitly state the upper interval at which the first or second dose of MMR vaccine could be administered, therefore it was not possible to distinguish between people vaccinated on or off the immunisation schedule. Therefore, in the analytical study, a person was assessed as vaccinated regardless of the interval between the first and second dose of the immunisation schedule. Since this is a trivalent vaccine, some patients may have been vaccinated for reasons other than MMR vaccination in childhood according to the national schedule (e.g. outbreak of measles in the population, for travel reasons).

Mumps cases are reported through a passive surveillance system, therefore under-reporting cannot be ruled out. Information on hospitalisation is usually recorded at the time of testing and hospitalisations may also have been underreported.

Our findings demonstrated that two doses of vaccine substantially and statistically significantly reduced the risk of any complications and the need for hospitalisation compared with unvaccinated patients. A protective effect of two-dose vaccination against mumps complications in general and against orchitis and meningitis in particular, and against hospitalisation for mumps was confirmed. Teenagers were the most affected age group in terms of incidence, but the highest complications and hospitalisations rates were reported in 35–44 years old. This study showed that the majority of mumps patients did not require hospitalisation. The most frequent complications were orchitis, meningitis, pancreatitis and encephalitis. Among the two-dose recipients, the proportion of complications increased gradually with the time interval after the second dose.

In the future, it will be desirable to evaluate aVE based on a new vaccination register or vaccination data from health insurance companies. To assess the burden of the disease, a seroepidemiological study seems suitable as the last nationwide serological survey of mumps antibodies was performed in 2013. Further monitoring and research are required to evaluate the new MMR vaccination schedule in order to assess if it has been more effective. We recommend continuing routine childhood mumps vaccination and maintaining high MMR coverage in CZ.