Sarcoidosis Rates in BCG-Vaccinated and Unvaccinated Young Adults: A Natural Experiment Using Danish Registers

Abstract

Objectives.

Sarcoidosis may have an infectious trigger, including Mycobacterium spp. The Bacille Calmette-Guérin (BCG) vaccine provides partial protection against tuberculosis and induces trained immunity. We examined the incidence rate (IR) of sarcoidosis in Danish individuals born during high BCG vaccine uptake (born before 1976) compared with individuals born during low BCG vaccine uptake (born in or after 1976).

Methods.

We performed a quasi-randomized registry-based incidence study using data from the Danish Civil Registration System and the Danish National Patient Registry between 1995 and 2016. We included individuals aged 25–35 years old and born between 1970–1981. Using Poisson regression models, we calculated the incidence rate ratio (IRR) of sarcoidosis in individuals born during low BCG vaccine uptake versus high BCG vaccine uptake, adjusting for age and calendar year (separately for men and women).

Results.

The IR of sarcoidosis was increased for individuals born during low BCG vaccine uptake compared with individuals born during high BCG vaccine uptake, which was largely attributed to men. The IRR of sarcoidosis for men born during low BCG vaccine uptake versus high BCG vaccine uptake was 1.22 (95% confidence interval [CI] 1.02–1.45). In women, the IRR was 1.08 (95% CI 0.88–1.31).

Conclusion.

In this quasi-experimental study that minimizes confounding, the time period with high BCG vaccine uptake was associated with a lower incidence rate of sarcoidosis in men, with a similar effect seen in women that did not reach significance. Our findings support a potential protective effect of BCG vaccination against the development of sarcoidosis. Future interventional studies for high-risk individuals could be considered.

INTRODUCTION

The pathogenesis of sarcoidosis involves a genetic predisposition combined with an environmental interaction, however the inciting factor or antigen exposure that initiates sarcoidosis remains unknown [1–7]. Prior work has suggested an infectious etiology could be the trigger, with Mycobacterium spp. and Propionibacterium acnes commonly implicated [8–14].

The Bacille Calmette-Guérin (BCG) vaccine is a live attenuated strain of Mycobacterium bovis that was developed to protect against tuberculosis (TB) and other mycobacterial infections [15]. Although the BCG vaccine was designed to prevent TB, it has been shown to potentially exert additional benefits, such as protection against other respiratory illnesses including Coronavirus disease 2019 (COVID-19), reduction in overall childhood mortality, enhancement of the influenza vaccine, and treatment, and/or prevention, of autoimmune disease [16–29].

Denmark started a national BCG immunization program in 1946, resulting in routine administration of the BCG vaccine to all children at the age of seven [30, 31]. This program began to phase out in 1983 (affecting those born in 1976) and officially ended in 1986, due to a substantial decrease in the prevalence of TB [30]. Given the evidence for a potential infectious trigger of sarcoidosis, including Mycobacterium, and the protection and long-lasting trained immunity conferred by BCG vaccination, there is evidence to believe that sarcoidosis rates may be reduced in individuals who received BCG vaccination. The objective of this study was to examine the incidence rate (IR) of sarcoidosis in individuals born during low BCG vaccine uptake compared with individuals born during high BCG vaccine uptake.

METHODS

Data Source and Study Design

The Danish National Health Service provides universal, tax-supported healthcare for all Danish residents [32]. We analyzed data from the Danish Civil Registration System and the Danish National Patient Registry (DNPR) between 1995 and 2016 [32]. The two registries are linkable through a unique personal identifier, which is assigned at birth or upon immigration. The DNPR contains nonpsychiatric hospitalizations since 1977 and outpatient clinic and emergency room visits since 1995 [33]. The DNPR was used to find incident sarcoidosis patients and their comorbid conditions. The Danish Civil Registration System was used to define date of birth, sex, date of death, and emigration status for each member of the study population [34]. We used changes in Denmark’s national BCG immunization policy as a natural experiment to conduct this registry-based incidence study. Our study relies on the fact that individuals born before or after 1976 have substantially different probabilities of receiving the BCG vaccination but are otherwise expected to have similar characteristics and balance of potential confounders. This quasi-random treatment assignment makes this study less prone to observed- and unobserved confounding, which is otherwise typical in observational studies [35].

We analyzed two different study populations. First, the total Danish population between 1995 and 2016 was used to study the IR of sarcoidosis by calendar year, sex, and age. Second, the effect of BCG vaccination on sarcoidosis incidence was analyzed in a sub-population born between 1970–1981, and these individuals were followed from age 25 to 35.

Exposure

We considered birth years between 1970–1975 as the high BCG vaccine uptake period and birth years between 1976–1981 as the low BCG vaccine uptake period. Even though BCG vaccination was compulsory by law until 1986, its application started to phase out some years earlier, with a precipitous decline in the rate of BCG vaccination occurring in 1983 (affecting those born in 1976, as the vaccine was administered at age seven) [30]. There is no Danish vaccination register covering our study period, thus we could not assess the person-level vaccination status in the population. Nevertheless, Rieckmann et al. reported the rate of vaccinated children by year of birth in a sub-population based on the Copenhagen School Health Record Register [30]. According to their study, the BCG vaccination rate decreased from roughly 90% to 70% for individuals born in 1970 to 1975, followed by a steep decline to roughly 20% for individuals born in 1976 (Supplementary Appendix Figure S1) [30].

Outcome Ascertainment

We identified incident sarcoidosis based on two or more International Classification of Diseases 10^th revision (ICD-10) codes for sarcoidosis (D86) registered in the DNPR and separated by 14 days or more. The date of sarcoidosis incidence was defined as the date of the first sarcoidosis diagnosis. Individuals who had a cancer diagnosis 6 months before or after their first diagnosis of sarcoidosis were excluded. This was done to minimize the risk of misclassification, as cancer is an important differential diagnosis to sarcoidosis due to similarities in clinical presentation and pathology.

Statistical Analysis

First, we calculated the IR of sarcoidosis in the Danish population between 1995 and 2016 by calendar year, sex, and age, reported as the number of sarcoidosis events per 100,000 person-years. We used the normal approximation of the Poisson distribution to calculate the 95% confidence intervals (CIs) [36].

As the incidence of sarcoidosis partly depends on age, we constructed observation periods covering the same age range (25–35 years) for individuals with high and low BCG vaccine uptake (Supplementary Appendix Figure S2). For each individual diagnosed with sarcoidosis, we collected information on sex, birth year, and date of diagnosis. We visualized the incidence of sarcoidosis in this age-balanced population by cumulative incidence probability curves and by plotting the IR of sarcoidosis by birth year and sex. Given the known differences in sarcoidosis incidence among men and women, the decision to stratify by sex was made a priori.

The number of sarcoidosis cases and person-time at risk were aggregated in the age-balanced population by birth year, calendar year, and sex. We fitted Poisson regression models to these data to estimate the incidence rate ratio (IRR) of sarcoidosis in individuals born during the low BCG vaccine uptake period versus individuals born during the high BCG vaccine uptake period, adjusting for age and calendar period (before or after 2008), separately for each sex. Second order polynomials were applied to describe the age dependency of sarcoidosis incidence (Supplementary Appendix Text S1 and Supplementary Appendix Table S1). We characterized the uncertainty of the IRR estimation with its 95% CI. We considered the estimate statistically significant if the 95% CI did not include one.

In a sensitivity analysis, the vaccination ratio of the Danish population—varying by birth year—was used in the modeling. The IRR estimates in individuals with low BCG vaccine uptake versus high BCG vaccine uptake are expected to be less than the IRR between non-vaccinated versus vaccinated individuals. To obtain a less biased estimate of this latter IRR, we fitted an alternative model with continuous vaccination ratios as explanatory variables. The vaccination ratios published by Rieckmann et al [30] were used between 1970–1976; after 1976 we assumed that the vaccination ratio dropped by 50% each year.

Statistical analyses were conducted using SAS version 9.4 (SAS Institute Inc., Cary, NC, USA). This study was reported to the Danish Data Protection Agency (record number 2016-051-000001/1880) by Aarhus University.

RESULTS

Sarcoidosis Incidence in the Total Danish Population

Between 1995 and 2016, there were 11,697 incidence cases of sarcoidosis (Table 1). The IR of sarcoidosis slightly increased over time, with a notable jump in cases in 2008 (Figure 1A). The sarcoidosis incidence depends strongly on age and sex and is highest among young males (Figure 1B).

Table 1.

Sarcoidosis cases between 1995–2016.

	Number of patients
Patients with sarcoidosis diagnosis (ICD-10 code: D86)	14495
Excluded due to cancer	799
Excluded due to having only one diagnosis ever	18
Excluded due to not having 14 days between diagnoses	1999
Final number of sarcoidosis cases	11697

Figure 1.

Sarcoidosis incidence rate by calendar year (A) and by sex (B).

Sarcoidosis versus Birth Year

The hypothesized causal relation of BCG vaccination, the sarcoidosis IR, and potential measured and unmeasured confounders is depicted in a directed acyclic graph (DAG) (Supplementary Appendix Figure S3). As it follows from the DAG, age and calendar year should be adjusted for in the regression analysis (Supplementary Appendix Text S2). Adjustment for age was partly achieved by study design: individuals were followed from age 25 to 35. The IR of sarcoidosis within this age range showed a slight upward tendency by birth year, with a noticeable jump in 1976—the year BCG vaccine uptake significantly declined (Figure 2A). The increased incidence of sarcoidosis after the birth year of 1976 was largely attributed to men (Figure 2B). The cumulative incidence probability curves showed the same pattern, with an increased incidence of sarcoidosis in male individuals born during low BCG vaccine uptake compared with male individuals born during high BCG vaccine uptake (Figure 3A). This was also observed in female individuals, but to a lesser degree (Figure 3B).

Figure 2.

Incidence rate of sarcoidosis by birth year between ages 25–35 (A) and by birth year and sex between ages 25–35 (B). The dashed line at birth year 1976 represents the year BCG vaccination precipitously declined.

Figure 3.

Cumulative incidence probability of sarcoidosis in men (A) and women (B).

Sarcoidosis versus BCG Vaccination

The sarcoidosis IR was 1.21 times higher (95% CI 1.02–1.44) in men during the time of low BCG vaccine uptake compared with men during the time of high BCG vaccine uptake (Model 1, Table 2). The estimated IRR was 1.08 for women (95% CI 0.88–1.32) with low BCG vaccine uptake versus high BCG vaccine uptake (Model 4, Table 2).

Table 2.

IRR of sarcoidosis in individuals with low BCG vaccine uptake versus high BCG vaccine uptake and in non-vaccinated vs vaccinated individuals.

Sex	Age	High vaccination rate				Low vaccination rate				Main analysis		Sensitivity analysis
		N	P-yr	Inc	IR	N	P-yr	Inc	IR	Model	IRR (95% CI)*	Model	IRR (95% CI)**
Men	25–35	220899	2384180	422	1.77	183726	1980054	463	2.34	1	1.21 (1.02–1.44)	7	1.26 (0.97–1.63)
Men	25–30	220899	1312450	204	1.55	183726	1090746	201	1.84	2	1.09 (0.87–1.36)	8	1.06 (0.78–1.44)
Men	31–35	216375	1071730	218	2.03	179645	889308	262	2.95	3	1.49 (1.10–2.03)	9	1.88 (1.25–2.83)
Women	25–35	212173	2298230	335	1.46	176549	1912295	304	1.59	4	1.08 (0.88–1.32)	10	1.09 (0.82–1.45)
Women	25–30	212173	1261949	189	1.50	176549	1050372	166	1.58	5	1.06 (0.84–1.33)	11	1.05 (0.75–1.49)
Women	31–35	208503	1036282	146	1.41	173547	861923	138	1.60	6	1.13 (0.76–1.66)	12	1.20 (0.62–2.31)

N = number at risk at the start of the period; P-yr = person-year, total time at risk in years; Inc = number of incident cases during the period; IR = incidence rate of sarcoidosis per 10,000 person-year.

^*IRR of sarcoidosis in individuals with low BCG vaccine uptake versus individuals with high BCG vaccine uptake;

^**IRR of sarcoidosis in non-vaccinated individuals versus vaccinated individuals.

The models use the assumption that the effect of BCG vaccination is the same for every age. Table 2 contains the estimates of the effect of BCG vaccination separately for younger and older age groups. The increase in sarcoidosis IR was more pronounced in the 31–35 age group for both men and women (Models 3 and 6, Table 2). The IRR for men was 1.09 (95% CI 0.87–1.36) between ages 25 and 30, compared with 1.49 (95% CI 1.10–2.03) for ages 30–35.

Sensitivity Analysis

The IRRs for non-vaccinated versus vaccinated individuals using an alternative model with continuous vaccination ratios as explanatory variables were similar to the previous estimates found between individuals with low BCG vaccine uptake versus high BCG vaccine uptake (Table 2). Although the point estimates did not change substantially, the variance of the estimates increased, which is reflected in the wider confidence intervals. In men aged 31–35 years, the IRR increased to 1.88 (95% CI, 1.25–2.83) (Models 7–12, Table 2).

DISCUSSION

In our study, we took advantage of a change in the BCG vaccination policy in Denmark to investigate the link between BCG vaccination in childhood and the development of sarcoidosis later in life. We showed an IR of sarcoidosis among male individuals who likely did not receive the BCG vaccine 1.21 times the rate in male individuals who likely did receive the BCG vaccine, which was mainly attributable to men above the age of 30 where the IRR increased to 1.49. For female individuals with low BCG vaccine uptake, the IR was 1.08 times the rate of individuals with high BCG vaccine uptake. This suggests that even in a young population in which incident sarcoidosis is relatively uncommon, there is an association between BCG vaccination and a lower incidence of sarcoidosis. This effect was most pronounced in men and did not reach significance in women. As seen in the cumulative incidence probability curves, there is increasing separation of the curves for men as age increases, and thus we would expect this effect to be even more pronounced with longer follow up.

From 1995–2016, we observed a steady increase in sarcoidosis incidence, with a notable rise in cases in 2008. These data are in line with a previously published report [37]. The increase in sarcoidosis incidence in 2008 may be due to a change in the Danish health care system that occurred in 2007, in which the number of municipalities was substantially reduced and the main administrative units were consolidated from 14 counties into five large regions [32, 33, 38]. As such, many small hospitals were closed and care became more centralized, which may have resulted in better diagnostic accuracy. In addition, fast-track (cancer) pathways were introduced in 2008, which likely resulted in more diagnostic procedures and accidental discovery of sarcoidosis cases [37]. We adjusted for calendar period (before or after 2008) in order to control for the fact that more patients in the high BCG vaccine uptake group were age 25–35 before 2008, and therefore followed for the outcome during that time. Without proper adjustment, this could potentially bias the results in favor of a lower incidence of sarcoidosis in the high BCG vaccine uptake group.

Given the evidence for a potential infectious trigger of sarcoidosis (including Mycobacterium), and the protection and long-lasting trained immunity conferred by BCG vaccination, there is a rational, biological basis to believe that sarcoidosis rates may be reduced in patients who received BCG vaccination. Prior work has demonstrated that an infectious etiology, such as Mycobacterium spp. or Propionibacterium acnes may trigger the development of sarcoidosis [12–14, 39–41]. In particular, Mycobacterium tuberculosis has been studied extensively. Mycobacterial RNA or DNA has been detected in up to 80% of pathology specimens, and an adaptive immune response to Mycobacterium has been demonstrated in sarcoidosis patients [9–11, 40]. However, Mycobacterium is not detected in all patient samples and conclusive evidence for a role in disease initiation and propagation is lacking.

The BCG vaccine, which provides partial protection against mycobacterial infection, was first administered to humans in 1921, and usage has varied significantly by country. In addition to preventing TB, the BCG vaccine has also been found to exert additional benefits, such as protection against other respiratory illnesses, reduction in overall childhood mortality, and enhancement of the influenza vaccine [16, 17, 22, 42, 43]. These effects are mediated by adaptive immune response cross-reactivity, as well as the potentiation of innate immune responses and epigenetic reprogramming of innate cell populations [18, 44]. More recently, it has been observed that countries with national BCG vaccination programs had lower prevalence of and mortality from coronavirus disease 2019 (COVID-19) [21, 23, 24]. This is hypothesized to be due to broad immune protection mediated by trained immunity [18, 20]. Lastly, BCG vaccination may protect against the development of, and/or help treat, autoimmune and immune-mediated diseases. In a clinical trial of patients with type 1 diabetes, BCG administration resulted in a transient increase in C-peptide blood levels [29]. BCG vaccination has also been shown to prevent the development of autoimmune diabetes in mice [25, 26, 45, 46]. In a study of patients with early signs of multiple sclerosis (MS), and in a second study of patients with relapsing-remitting MS, BCG vaccination was shown to provide benefit by reducing the rate of development of MS and by preventing progression of brain lesions [27, 28].

Several prospective studies have examined the potential protective effect of BCG vaccination against the development of sarcoidosis [47, 48]. This includes a vaccine trial conducted from 1950–1952, in which 54,239 participants, nearly all aged 14 to 15 years and living in England, were randomly assigned to receive the BCG vaccine (13,598 participants), vole-bacillus vaccine (5,817 participants), or were left unvaccinated (12,867 participants). The participants were followed until age 27 with annual chest radiographs, tuberculin tests, postal inquiries, and home visits until 1960, and with routine annual postal inquiries thereafter. During follow up, 52 participants were diagnosed with sarcoidosis (20 of whom had positive tuberculin tests at study entry and were followed without intervention). The mean annual incidence over the follow up period was lower in the two vaccinated groups (0.79 per 10,000 for BCG-vaccinated participants and 0.57 per 10,000 for vole-bacillus-vaccinated participants) than in the unvaccinated group (0.98 per 10,000), although this difference did not reach statistical significance. The authors concluded that tuberculosis vaccination at the age of 14 did not appear to protect against the development of sarcoidosis. This study did not report the case burden among BCG-vaccinated participants by sex, but the overall rate of sarcoidosis across all groups was lower in men aged 15–25 (0.39 per 10,000) than women aged 15–25 (1.28 per 10,000). Given our results suggesting a differential effect of BCG vaccination in men against the development of sarcoidosis, the lower incidence of sarcoidosis seen in men in this BCG vaccination trial is intriguing, but inconclusive.

The basis for a differential effect of BCG vaccination in men and women is unclear, but sex-differences after BCG vaccination are well described. In a study of all-cause mortality, there was a difference during the neonatal period between males and females, with a strong protective effect seen in males only during the first weeks after life [49]. Similarly, in a case-control study conducted in Kenya, community acquired pneumonia rates were significantly reduced in BCG-vaccinated males but not females [50]. A recent study showed a protective effect of BCG vaccination against the development of COVID-19 in men but not women (ORs 0.13 and 1.04, respectively) [23]. Lastly, BCG vaccination in healthy volunteers has been shown to reduce systemic inflammation in males only [51]. It is conceivable that BCG vaccination differentially affects men by redirecting them from a natural state of being at higher risk of developing sarcoidosis during the ages studied, to a state of being at lower risk compared with women.

Our study has several strengths. To our knowledge, this was the first study in over 65 years to evaluate the relationship between BCG vaccination and sarcoidosis rates [47, 48]. We were able to perform a natural experiment, examining the pre- and post-universal BCG vaccination time periods, which minimized confounding and allowed for stronger causal interpretation of our results. We studied a population all derived from the same country and with a relatively high incidence of sarcoidosis. We utilized a large, high quality, nationwide database encompassing over 8 million individuals.

LIMITATIONS

Our study has several limitations. First, due to the nature of this registry-based study, there may be uncontrolled confounding. However, we utilized a natural experiment framework, in which we compared people born between 1970–1975 with people born between 1976–1981. This design should result in a relatively equal distribution of many of the known and unknown confounders. Despite this, the lack of data on certain variables such as occupation, BMI, or smoking, all of which can be associated with sarcoidosis, could have affected the results. Environmental factors such as pollution, socioeconomic factors, and other changes in Danish society between the two time periods could also account for different exposures relevant to developing sarcoidosis, and we were unable to directly control for these. Second, the average age of sarcoidosis onset is 40–55 years, which may have limited the results of this study, as our study examined people between the ages of 25 and 35 [52]. However, given the relatively high prevalence of sarcoidosis in Denmark, we were still able to detect a meaningful number of incident sarcoidosis cases. Third, we began follow up in 1995 after ICD-10 codes had been fully implemented, and thus it is conceivable that we included some prevalent cases of sarcoidosis that were diagnosed at a very young age, before 1995. Although not truly incident, these prevalent cases (which would mistakenly be identified as incident) would still be relevant to the outcome and overall objective of the study. Fourth, we observed an increase in sarcoidosis incidence in 2008, which may be due to a change in the Danish health care system that occurred in 2007. Although we adjusted for calendar period (before or after 2008) in our analyses, this change to the healthcare system may have affected our results.

CONCLUSIONS

In this quasi-randomized registry-based incidence study, we found that the time period with high BCG vaccine uptake was associated with a lower incidence rate of sarcoidosis in men, with a similar but less pronounced effect seen in women. This could be due to enhanced immunity against a pathogen, such as Mycobacterium spp., or due to a reduction in systemic inflammation, both of which are factors that might contribute to the development of sarcoidosis. Prior work has demonstrated sex-differences in these effects, which might explain why the benefit is only significant in males [23, 49–51]. These data support a potential protective effect of BCG vaccination against the development of sarcoidosis, and future interventional studies for high-risk individuals could be considered.