Cécile Kremer

BACKGROUND: With a notification rate of 44.9 per million population in 2024, Belgium was among the countries with the highest measles rate of the European Union/European Economic Area countries. Although the coverage of the first dose of measles–mumps–rubella (MMR) vaccination in Flanders was 96% in 2020, two-dose coverage was below 95%. To minimise the risk of sustained measles transmission in vulnerable settings, such as primary schools with low vaccination coverage or daycares, it is important to quarantine susceptible contacts of a measles case. AIM: We aimed to identify policies that balance measles outbreak control with societal as well as educational impact of quarantining children. METHODS: Using a simulation model, we evaluated the impact of different quarantine strategies for measles in primary school and daycare settings in Flanders, Belgium. RESULTS: We found that shortening the quarantine period from 21 to 18 days only moderately (e.g. 16–37% in primary school) increased the risk of a subsequent infection wave, and the final proportion of later-generation cases remained at most 3.2% of the school or daycare. In settings with a low vaccination coverage (e.g. 20%), multiple independent introductions or a low diagnosis rate (e.g. only half of symptomatic children going to a doctor), the addition of post-exposure vaccination can further reduce transmission risk. CONCLUSION: We found that quarantine duration could be shorter than the presumed maximal incubation period of 21 days, without substantially increasing the risk of onward transmission. As such, acceptability and compliance with mandated quarantine periods might also increase.

Monkeypox virus (MPXV) Clade Ib, a new MPXV subclade associated with sustained human-to-human transmission, emerged in eastern DRC in late 2023. Early spread mainly occurred among the adult population, primarily through sexual contact. We investigate how key epidemiological characteristics of the new MPXV Clade Ib outbreak in the DRC differ by transmission mode. We estimate the mean serial interval to be 8.57 days (95% CI 7.15 − 9.91 days). The serial interval is approximately four days shorter for putative sexual compared to non-sexual transmission. We estimate the mean incubation period to be 9.56 days (95% CrI 8.22 − 11.02 days), and approximately two days shorter for putative sexual compared to non-sexual transmission. These observed differences in serial interval and incubation period estimates by transmission mode suggest that the context in which an outbreak occurs is an essential factor to consider when devising public health policies.

Monkeypox virus (MPXV) Clade Ib, a new MPXV subclade associated with sustained human-to-human transmission, emerged in eastern DRC in late 2023. Early spread mainly occurred among the adult population, primarily through sexual contact. We investigated how key epidemiological characteristics of the new MPXV Clade Ib outbreak in the DRC differ by transmission mode. We estimated the mean serial interval to be 8.57 days (95% CI 7.15 – 9.91 days). The serial interval was approximately four days shorter for putative sexual compared to non-sexual transmission. We estimated the mean incubation period to be 9.56 days (95% CrI 8.22 – 11.02 days), and approximately two days shorter for putative sexual compared to non-sexual transmission. These observed differences in serial interval and incubation period estimates by transmission mode suggest that the context in which an outbreak occurs is an essential factor to consider when devising public health policies.

The goal of tracing, testing, and quarantining contacts of infected individuals is to contain the spread of infectious diseases, a strategy widely used during the COVID-19 pandemic. However, limited research exists on the effectiveness of contact tracing, especially with regard to key performance indicators (KPIs), such as the proportion of cases arising from previously identified contacts. In our study, we analyzed contact tracing data from Belgium collected between September 2020 and December 2021 to assess the impact of contact tracing on SARS-CoV-2 transmission and understand its characteristics. Among confirmed cases involved in contact tracing in the Flemish and Brussels-Capital regions, 19.1% were previously identified as close contacts and were aware of prior exposure. These cases, referred to as ‘known’ to contact tracing operators, reported on average fewer close contacts compared to newly identified individuals (0.80 versus 1.05), resulting in fewer secondary cases (0.23 versus 0.28). Additionally, we calculated the secondary attack rate, representing infections per contact, which was on average lower for the ‘known’ cases (0.22 versus 0.25) between December 2020 and August 2021. These findings indicate the effectiveness of contact tracing in Belgium in reducing SARS-CoV-2 transmission. Although we were unable to quantify the exact number of prevented cases, our findings emphasize the importance of contact tracing as a public health measure. In addition, contact tracing data provide indications of potential shifts in transmission patterns among different age groups associated with emerging variants of concern and increasing vaccination rates.

Background: In countries with mature generalized HIV epidemics such as Uganda, there are still groups of individuals that are disproportionately affected. Among the key populations in Uganda are fishing communities, which make up about 10% of the population. Compared to the general population, HIV prevalence and incidence among individuals living in these communities is high. This high HIV burden has been attributed to several factors including limited access to prevention and treatment services as well as ongoing high-risk sexual behaviour. Methods: We investigated the impact of combined HIV prevention interventions on HIV transmission dynamics in high-risk fishing communities in Uganda using a deterministic compartmental model. The model was calibrated to seroprevalence data from a census performed in 2014. To account for remaining uncertainty in the calibrated model parameters, 50 000 simulated scenarios were modelled to investigate the impact of combined prevention interventions. Results: The projected HIV incidence decreased from 1.87 per 100 PY without intervention scale-up to 0.25 per 100 PY after 15 years (2014–2029) of intervention scale-up. A potential combination achieving this 87% reduction in incidence over 15 years in Ugandan FCs included condom use in about 60% of sexual acts, 23% of susceptible men circumcised, 87% of people living with HIV aware of their status, 75% of those on ART, and about 3% of susceptible individuals on oral PrEP. Uncertainty analysis revealed relative reductions in incidence ranging from 30.9 to 86.8%. Sensitivity analyses suggested that condom use and early ART were the most important interventions. Conclusion: Reducing HIV incidence, as well as prevalence and AIDS-related mortality, in these high-risk fishing communities in Uganda is attainable over 15 years with a combination prevention package. Our projected intervention coverage levels are well within the national targets set by the Uganda government and enable coming close to reaching the UNAIDS 95-95-95 targets to end AIDS as a public health threat by 2030.

Mathematical modeling studies have shown that repetitive screening can be used to mitigate SARS-CoV-2 transmission in primary schools while keeping schools open. However, not much is known about how transmission progresses within schools and whether there is a risk of importation to households. In this study, we reconstructed outbreaks observed during a prospective study in a primary school and associated households in Liège (Belgium) during the academic year 2020-2021. In addition we performed a simulation study to investigate how the accuracy of estimated weekly positivity rates in a school depends on the proportion of a school that is sampled in a repetitive screening strategy. We found that transmission occurred mainly within the school environment and that observed positivity rates are a good approximation to the true positivity rate, especially in children. This study shows that it is worthwile to implement repetitive testing in school settings, which in addition to reducing infections can lead to a better understanding of the extent of transmission in schools during a pandemic and importation risk at the community level.

Background: Contact tracing aims to prevent onward transmission of infectious diseases and data obtained during tracing provide unique information on transmission characteristics. A key performance indicator that has been proposed to evaluate contact tracing is the proportion of cases arising from known contacts. However, few empirical studies have investigated the effectiveness of contact tracing. Methods: Using data collected between September 2020 and December 2021 in Belgium, we investigated the impact of contact tracing on SARS-CoV-2 transmission. We compared confirmed cases that were previously identified as a close contact to those that were not yet known, in terms of their traced contacts and secondary cases as well as the serial interval. In addition, we established contact and transmission patterns by age. Findings: Previously traced, hence ‘known’, cases comprised 20% of all cases and they were linked to relatively fewer close contacts as well as fewer secondary cases and a lower secondary attack rate compared to cases that were not already known. In addition we observed a shorter serial interval for ‘known’ cases. There was a relative increase in transmission from children to adults during circulation of the Delta and Omicron variants, without an increase in the extent of contact between these age groups. Interpretation: These results suggest that contact tracing in Belgium has been effective in reducing onward transmission and that individuals aware of their exposure to SARS-CoV-2 seemed more reserved in their social contact behaviour. Data from a reference period or region are needed to measure the impact of contact tracing in terms of the number of cases and deaths averted.

Serial Intervals for SARS-CoV-2 Variants, Belgium

Background: In countries with mature generalized HIV epidemics such as Uganda, there are still groups of individuals that are disproportionately affected. These most at risk populations (MARPs) have high HIV prevalence compared to the general population. Among the MARPs in Uganda are fishing communities, which make up about 10% of the population. Methods: We investigated the impact of combined HIV prevention interventions on HIV transmission dynamics in high-risk fishing communities in Uganda using a deterministic compartmental model. The model was calibrated to seroprevalence data from a census performed in 2014. To account for remaining uncertainty in the calibrated model parameters, an additional 50 000 simulated scenarios were modelled to investigate the impact of combined interventions. Results: The projected HIV incidence decreased from 1.87 per 100 PY without intervention scale-up to 0.25 per 100 PY after 15 years (2014-2029) of intervention scale-up. A potential combination achieving this 87% reduction in incidence included condom use in about 60% of sexual acts, 23% of susceptible men circumcised, 87% of people living with HIV aware of their status, 75% of those on ART, and about 3% of susceptible individuals on oral PrEP. Uncertainty analysis revealed relative reductions in incidence ranging from 30.9% to 86.8%. Sensitivity analyses suggested that condom use and early ART were the most important intervention components. Conclusions: Reducing HIV incidence, and consequently prevalence and AIDS-related mortality, in these high-risk fishing communities in Uganda is attainable in the long-run with a combination prevention package. Our projected intervention coverage levels are well within the national targets set by the Uganda government and close to reaching the UNAIDS 95-95-95 goal to end AIDS by 2030.