Seroprevalence in the Nationwide Sample of the Pienter2 Study
The overall seroprevalence in the nationwide sample was 91% (95% CI 90–92). Seroprevalence of maternal mumps antibodies declined rapidly to 2.5% at the age of 5 months (95% CI 0–8) (). After the first vaccination at 14 months there was a rapid increase of seroprevalence in the subsequent age groups to 90% (95% CI 85–95) at the age of 2 years. Thereafter it declined gradually to 71% (95% CI 62–81) at the age of 8 years. The administration of the second MMR vaccination at the age of 9 years induced an increase in seroprevalence to 96% (95% CI 92–100) at 10 years of age. Seroprevalence in the twice-vaccinated cohorts decreased gradually to 87% (95% CI 82–92) in the age group of 19–21 years. Participants born before introduction of MMR-vaccination (1987) who were once or twice vaccinated (aged 22–27 years) had a seroprevalence of 91%. Among non-vaccinated individuals, the seroprevalence gradually increased up to 98% in the oldest age groups.
GMC and seroprevalence of the national sample of the Pienter2 study.
GMCs in the Nationwide Sample of the Pienter2 Study
The overall GMC in the nationwide sample was 198 RU/ml (95% CI 190–206). The GMC was lower among males than females (183 RU/ml (95% CI 172–194) and 214 RU/ml (95% CI 204–224), respectively, p<0.001). In the first 6 months of life, the GMC of maternal antibodies declined rapidly from 91 RU/ml (95% CI 37–219) just after birth to 5 RU/ml (95% CI 2–11) (). After the first MMR vaccination at 14 months of age, the GMC increased to 129 RU/ml (95% CI 112–149) at 2 years of age and then declined gradually to 90 RU/ml (95% CI 72–113) by the age of 7 years. The second MMR vaccination around the age of 9 years boosted the GMC to 220 RU/ml (95% CI 187–259), after which the GMC decreased again to 142 RU/ml (95% CI 111–180) by the age of 11 years. Thereafter GMC remained constant between 130–140 RU/ml up to the age of 20 years. Among those aged 22 to 27 years, representing birth cohorts who were not (fully) vaccinated, the GMC showed a sharp increase to about 250 RU/ml. In the older non-vaccinated cohorts the GMC remained constant up to the age of 50–54 years. Thereafter the GMC gradually increased up to almost 350 RU/ml in the oldest age group.
Comparison of the Pienter1 and 2 Studies
The maternal mumps antibody levels decreased faster in the Pienter2 study than in the Pienter1 study (). For the once vaccinated cohorts (2–8 years) antibody levels increased in parallel upon the first MMR vaccination, but the levels after vaccination were slightly lower in the Pienter2 study. The second MMR vaccination induced a further increase of antibody levels, which was most striking for the Pienter2 study. As expected, the rise in GMC to a level around 250 RU/ml for the non-vaccinated individuals in the Pienter2 study showed a 10-year shift in concordance with the time period between the two studies. This rise in GMC highlights the difference between the vaccinated and naturally infected cohorts starting in the Pienter1 study from the age of 10–11 years and in the Pienter2 study from 20–21 years. In the unvaccinated naturally infected cohorts above 30 years of age, GMCs were higher in the Pienter2 study compared with the Pienter1 study.
GMC of the national sample of the Pienter1 and Pienter2 study.
The corresponding seroprevalence in the different age cohorts of the Pienter1 and Pienter2 study () indicate that the results of both studies were highly comparable, with a few exceptions. A significant lower seroprevalence of maternal antibodies was observed in the Pienter2 study among the 4–5 months (p
0.0006) and 6–9 months (p
0.012) age groups (). Furthermore, we noticed a significantly lower seroprevalence among participants aged 6, 7 and 8 years in the Pienter2 study as compared to the Pienter1 study (p
0.0016, 0.0006 and <0.0001, respectively). Moreover in the 7-years age group from the Pienter2 study, the seroprevalence for boys (61% (95% CI 49–72)) appeared to be significantly (p
0.003) lower than for girls (85% (95% CI 75–95)) (). In contrast, no such gender-specific difference for these age groups was observed in the Pienter1 study. Also, a lower seroprevalence was observed in the Pienter2 study for the 15–21 year old age group, which represented antibody levels acquired through vaccination instead of natural mumps infection. Seroprevalence in this age group was 87% in the Pienter2 study, while this was 94% in the Pienter1 study ().
Waning Immunity in the Pienter2 Study
After both the first and second MMR vaccination, a significant decline in antibody concentration could be observed in the Pienter2 study (, p
0.02 and p<0.0001 respectively). Characteristic for the decline after the first vaccination was the constant decrease in ln-transformed antibody concentration with 0.067 RU/ml per year after vaccination. The initial rise induced by the second vaccination was succeeded by an exponential decrease in ln-transformed antibody concentration in the first 3–4 years. Importantly, in the years thereafter antibody concentrations remained constant around 138 RU/ml.
Mumps antibody persistence after vaccination.
Risk Factors in the Nationwide Sample of the Pienter2 Study
Multivariable analysis indicates that younger age groups will have a higher risk at being susceptible for an infection with mumps compared to the oldest, naturally infected age groups because of their lower seroprevalence (). Also, gender, the number of vaccinations and urbanization degree were found to be significant risk factors. A higher risk of being susceptible was observed for males as compared to females (OR
1.4 (95% CI 1.1–1.6)), having had no vaccinations as compared to having had two vaccinations (OR
4.6 (95% CI 3.0–7.1)) or living in a region with a moderate urbanization degree as compared to living in a region with a high urbanization degree (OR
Potential risk factors for mumps seronegativity in the Pienter2 study with corresponding crude and adjusted odds ratios (ORs) and 95% confidence intervals (CIs): results of univariable and multivariable logistic regression analysis.
Low Vaccination Sample
For the orthodox reformed individuals in the Pienter2 study, the seroprevalence in the younger age cohorts was low, when compared to the same age cohorts of the national sample. By the age of 20–24 years, seroprevalence reached a level of 86% (95% CI 73–98), while a level of 79% (95% CI 73–86) was already reached by the age of 1–4 years in the non-orthodox reformed individuals (). This difference in seroprevalence reflected the degree of vaccination coverage in the two groups. In the non-orthodox reformed group, vaccination coverage amounted to around 77%, thereby following the trend of the national sample, while in the strictly reformed group, vaccination coverage reached a maximum of 33%.
Seroprevalence of the low vaccination coverage sample.
The seroprevalence of the LVC sample from the Pienter1 study provided a similar pattern, but seroprevalence for the strictly reformed group significantly increased within the age group of 5–9 years. This rise was delayed in the Pienter2 study by approximately 10 years with a rise starting within the age group of 10–14 years (). For the age cohorts older than 24 years, seroprevalence remained above 90% in as well the Pienter1 as the Pienter2 study for both groups of the LVC.