
==== Front
Pediatr Infect Dis J
Pediatr Infect Dis J
INF
The Pediatric Infectious Disease Journal
0891-3668
1532-0987
Lippincott Williams & Wilkins Hagerstown, MD

39105529
PIDJ-24-203
00020
10.1097/INF.0000000000004487
3
Vaccine Reports
Impact of the COVID-19 Pandemic on Measles Vaccination Coverage and Estimated Catch-up Efforts for Serbia
Burgess Colleen MS colleen.burgess@merck.com
*
Lisul Bogdan DMD, PhD bogdan.lisul@merck.com
†
Pawaskar Manjiri PhD manjiri.pawaskar@merck.com
*
Petigara Tanaz PhD tanaz.petigara@merck.com
*
Murtagh Janice PhD janice.murtagh@merck.com
‡
Kanazir Milena MD, PhD kanazir_milena@batut.org.rs
§
Loncarevic Goranka MD, PhD afp@batut.org.rs
§
Carias Cristina PhD *
* From the Merck & Co., Inc., Value and Implementation - Outcomes Research, Rahway, NJ
† MSD Serbia, Global Medical Affairs SEE, Belgrade, Serbia
‡ MSD Ireland, Global Medical and Scientific Affairs, Dublin, Ireland
§ Institute of Public Health of Serbia “Dr Milan Jovanovic Batut,” Belgrade, Serbia.
Address for correspondence: Cristina Carias, PhD, Merck & Co., Inc., 351 N Sumneytown Pike, North Wales, PA 19454. E-mail: cristina.da.silva.carias@merck.com.
16 9 2024
10 2024
43 10 10111017
31 5 2024
Copyright © 2024 The Author(s). Published by Wolters Kluwer Health, Inc.
2024
https://creativecommons.org/licenses/by-nc-nd/4.0/ This is an open-access article distributed under the terms of the Creative Commons Attribution-Non Commercial-No Derivatives License 4.0 (CCBY-NC-ND), where it is permissible to download and share the work provided it is properly cited. The work cannot be changed in any way or used commercially without permission from the journal.

Background:

Measles is highly infectious, requiring ≥95% vaccine coverage rate (VCR) to prevent outbreaks. This study aimed to understand the impact of the COVID-19 pandemic on routine measles-containing vaccine (MCV) VCRs in Serbia and estimate national and regional catch-up vaccination required to prevent outbreaks.

Methods:

A multiplier model was used to calculate annual MCV dose 1 (MCV1) and MCV dose 2 (MCV2) VCRs for children 1-6 and 6-12 years of age, respectively, for 2011-2022. Postpandemic (2023–2024) VCRs were modeled. The numbers of administered doses were compared to prepandemic and postpandemic, and monthly catch-up rates were calculated for 12-, 18- and 24-month campaigns.

Results:

Between prepandemic and pandemic periods, national MCV1 VCR decreased from 88% to 81%, while MCV2 VCR decreased from 92% to 89%, corresponding to 20,856 missed MCV1 and 8760 missed MCV2 doses. Assuming national VCRs returned to prepandemic levels post-2022, 18% of children 1-6 years of age and 11% of children 6-12 years of age would have missed their MCV1 and MCV2 doses, respectively, by 2024. To catch up missed doses under this scenario, most regions would require monthly catch-up rates of 25%, 16% or 12% for MCV1 and 14%, 9% or 7% for MCV2, considering 12-, 18- or 24-month campaigns, respectively.

Conclusions:

The pandemic negatively impacted MCV VCRs in Serbia, leaving a sizeable proportion of children with missed doses. Significant catch-up efforts are required to recover VCRs to prepandemic levels and avoid future measles outbreaks, with increased monthly administration rates versus those in prepandemic periods.

measles
measles-containing vaccine
vaccination coverage
vaccination catch-up
Serbia
Merck & Co., Inc., Rahway, NJ, USAColleen BurgessOPEN-ACCESSTRUE
SDCT
==== Body
pmcINTRODUCTION

Immunization is an effective preventive measure that has reduced morbidity and mortality from infectious diseases across the world.1 In Serbia, mandatory immunization dates back to 1839 when smallpox vaccination was introduced.2 The Institute of Public Health of Serbia, along with a network of 24 regional institutes and public health centers, coordinates and supervises immunizations in 160 health centers and 55 maternity hospitals.3 To attend preschools, schools and institutions for children without parental care, children are required to be vaccinated according to the immunization schedule. However, despite vaccination against measles being a requirement for school attendance, measles epidemics persist in Serbia.4

Measles is one of the most infectious vaccine-preventable diseases, and a vaccine coverage rate (VCR) of ≥95% is required to prevent outbreaks through population immunity according to guidance from the World Health Organization (WHO).5–7 Measles vaccination was introduced into the Mandatory Immunization Program in Serbia in 1971, with measles and mumps vaccination ensuing in 1981 and measles, mumps and rubella (MMR) vaccination following in 1993.8 After the start of the immunization program, measles cases still occurred cyclically (epidemic waves every 3–5 years) but with significantly fewer cases compared to prevaccination periods.9 Four epidemic waves were recorded from 1986 to 1997 [incidence rate (IR), 42.9–150.5/100,000 population].9 The measles IR in Serbia has consistently decreased since 1998, reaching the lowest recorded rate of 0.02/100,000 population in 2005-2006.9 However, a large measles epidemic spread through Europe between 2010 and 2020, and new cases were recorded in Serbia in 2010-2011 (IR, 4.93/100,000 population), 2014-2015 (IR, 5.35/100,000 population) and 2017-2018 (IR, 70.3/100,000 population), with 15 fatalities in total.10,11

Although measles vaccination was introduced in 1971, measles VCR remained below 85% in Serbia up to 1996.9 Since then, VCRs have improved, and at the end of 2006, an action plan for the elimination of measles, rubella and congenital rubella syndrome was adopted in Serbia, and active surveillance for measles and rubella was established in 2009.12 Between 2006 and 2009, MMR VCRs reached or exceeded the target rate of 95%.9 Since 2010, however, MMR dose 1 VCRs have decreased and vary between 74.6% and 93.4%.11

Given its history of measles transmission, Serbia is classified by the WHO as a country where measles is endemic.8 Due to challenges in maintaining sufficiently high VCRs, achieving elimination has been postponed to 2030, versus the initial goal of 2010.13 The COVID-19 pandemic had an additional negative impact on the implementation of immunization programs in many countries, including Serbia.14 Globally, the first dose of measles VCR dropped to 81% in 2021, the lowest since 2008, meaning that 24.7 million children missed their first measles vaccine dose in 2021 (an additional 5.3 million missed doses compared to 2019).15 Additionally, 14.7 million children missed their second dose.15 Given the high transmissibility of measles, any decrease below the WHO-recommended rate increases the likelihood of measles outbreaks and measles-associated morbidity and mortality. A measles outbreak in Serbia started after the COVID-19 pandemic, with at least 50 cases reported between January 1 and July 13, 2023, demonstrating the potential impact of missed vaccinations during the pandemic.16

This study aimed to characterize the impact of the COVID-19 pandemic on routine measles-containing vaccine (MCV) VCRs in Serbia and estimated the national and regional catch-up efforts required to ensure population immunity and minimize the risk of further outbreaks. This study will be relevant for policymakers, public health practitioners, infectious disease modelers and healthcare providers to assess the COVID-19 pandemic impact and understand the need for recovery efforts.

METHODS

Following a previous COVID-19 pandemic impact assessment, a multiplier model was used to calculate the VCR for MCV dose 1 (MCV1; scheduled at 12–15 months) and MCV dose 2 (MCV2; scheduled at 6–7 years).17,18 The MCV1 and MCV2 VCRs thus calculated were specific to particular year-of-birth cohorts. We then estimated the VCR across specific age cohorts (1–6 and 6–12 years) to understand how VCRs compared to the WHO-recommended thresholds (95% for MCV1 and 90% for MCV2).6 We quantified the need for catch-up vaccination at national and regional (25 regions; Figure, Supplemental Digital Content 1, http://links.lww.com/INF/F662) levels, calculated as the percent increase in doses that would have to be administered on top of routine vaccination to bridge the immunization gap incurred during the pandemic.

Model Inputs

The MCV dose schedule for children 12-15 months and 6-7 years of age and the number of MCV1 and MCV2 doses administered in the years 2011-2022 were provided by The Institute of Public Health of Serbia.11 The number of planned doses was based on the projected number of children that required vaccination. This number included children in the birth cohort from the previous year, children who missed their first dose the year before, and children who missed their vaccination at 12 months old in a certain year.

Calculations

Yearly MCV VCRs by Birth Cohort for Prepandemic and Pandemic Years

We calculated the annual MCV1 and MCV2 VCRs nationally and across the 25 Serbian regions for children 12-15 months and 6-7 years of age, respectively. Annual VCRs were calculated for 2011-2022 by dividing the number of administered doses by the number of planned doses for each year.11,19 To calculate the average VCR for the prepandemic period, MCV1 and MCV2 VCRs were averaged over the prepandemic years. To understand the impact of the pandemic, we also calculated the difference in VCRs between prepandemic years (2011–2019) and 2022.

Projected MCV VCRs for Children 1-6 and 6-12 Years of Age for Postpandemic Years

To compare VCRs with WHO-recommended thresholds, we estimated the VCRs for children 1-6 and 6-12 of age for 2023 and 2024. MCV1 VCR for children 1-6 of age (12–71 months) was calculated per year as the average of the VCRs for each birth cohort at the time they were 12-15 months, weighted by the size of the respective birth cohort, as proxied by the planned vaccinations for that birth cohort. That is, the MCV1 VCR in 2023 for children 1-6 years of age corresponded to the weighted average of the VCRs of children born between 2016 and 2022, where the weights were the number of planned vaccinations between 2016 and 2022 reported for the respective cohort. MCV2 VCR among children 6-12 years of age (72–143 months) was similarly calculated based on the weighted average of the annual MCV2 VCRs and planned doses between 6 and 12 years (Fig. 1). We modeled the postpandemic VCRs of children receiving MCV doses under 2 scenarios: (1) the VCR for children born in 2023 and 2024 returned to average prepandemic levels and (2) the VCR remained the same as that in 2022.

FIGURE 1. Calculation of MCV1 (A) and MCV2 (B) cohort vaccination coverage among birth cohorts 1-6 and 6-12 years of age, respectively.

Catch-Up Vaccination Estimations

We defined the catch-up rate as the number of additional doses that needed to be administered as a percentage of the baseline vaccine doses administered, where the baseline number of doses varied in different scenarios. The number of additional doses that needed to be administered was equal to the number of missed doses divided by the number of months the vaccination campaign lasted. The catch-up rate provided a measure of the potential increase in capacity required in Serbia, nationally and in specific regions, to vaccinate not only the birth cohort but also the children that missed their vaccines during the pandemic.

To calculate the number of doses missed during the pandemic, the total doses administered in 2020-2022 were compared to the doses that would have been administered over that same time period, given the prepandemic monthly average doses administered (calculated based on historical data from The Institute of Public Health of Serbia, averaged over 2011–2019).11 We then calculated the catch-up rate by dividing the missed doses by the duration of a catch-up campaign, considering 12-, 18- and 24-month catch-up campaigns and an assumed ongoing monthly VCR (either returning to average prepandemic VCR or maintaining the 2022 VCR).

For the regional analyses, we then stratified the regions into low, medium, high and highest priorities based on the monthly catch-up rate requirements over a 12-month period while returning to average prepandemic VCRs. For MCV1, low priority was defined as a 0%-15% catch-up rate required, medium priority as 16%-30%, high priority as 31%–45% and highest priority as ≥46%. For MCV2, low priority was defined as a 0%-15% catch-up rate required, medium priority as 16%-30% and highest priority as ≥31%.

As the catch-up rates calculated may not be logistically feasible, we also estimated the number of months required to return VCRs to prepandemic average rates, assuming lower and standardized catch-up rates between 5% and 30% and different underlying VCRs for 2023 and 2024: (1) a return of VCRs to average prepandemic levels and (2) MCV1 and MCV2 VCRs equal to 2022 VCR. For MCV1, the number of months required to return to WHO-recommended rates was also calculated (this was not calculated for MCV2 as the average prepandemic MCV2 VCR exceeded the WHO-recommended rate).

RESULTS

Yearly MCV VCRs by Birth Cohort for Prepandemic and Pandemic Years

The national annual proportion of children receiving their MCV1 decreased from 88% in 2019 to 81% in 2022, while, for MCV2, the national annual VCR decreased from 92% in 2019 to 89% in 2022.

The number of regions with MCV1 VCR ≥90% decreased from 25/25 in 2011 to 15/25 in 2019 and subsequently to just 5/25 in 2022 (Fig. 2; Figure, Supplemental Digital Content 2, http://links.lww.com/INF/F663). Compared to average prepandemic rates, nearly all regions experienced decreased MCV1 VCRs in 2022 (Fig. 3A). From the prepandemic to the pandemic period, regional VCRs in most regions dropped by 2%-17%, while the VCR in 2 regions (Raska and Toplica) dropped by >20%. In three regions (Bor, Pcinja and North Banat), the MCV1 VCR increased by 1%-6% (Fig. 3A).

FIGURE 2. Number of Serbian regions with an annual MCV1 VCR ≥90% from 2011 to 2022.

FIGURE 3. Regional differences in the yearly birth cohort VCRs between the prepandemic period (2011–2019) and 2022 for MCV1 (A) and MCV2 (B). A total of 25 Serbian regions. VCR data for the region of Kosovo and Metohija are not available. The change in MCV1 (MCV2) VCR was defined as the difference between the average prepandemic MCV1 (MCV2) VCR from 2011 to 2019 and the MCV1 (MCV2) VCR in 2022. To calculate the average yearly VCRs for the prepandemic, MCV1 and MCV2 VCRs were calculated by year and then averaged over the prepandemic period.

There were no differences in MCV2 VCR in the postpandemic period. The number of regions with MCV2 VCR ≥90% was 24/25 in 2011 and 2019. In 2022, MCV2 VCR was ≥90% in 19/25 regions and 80%-89% in 6/25 regions (Figure, Supplemental Digital Content 2, http://links.lww.com/INF/F663). In comparison to prepandemic rates, in 2022, VCRs in 14 regions had dropped by 1%-6%, one region (Toplica) maintained the same VCR and 10 regions increased up to 11% (Fig. 3B).

Projected MCV VCRs for Children 1-6 and 6-12 Years of Age for Postpandemic Years

MCV1

We calculated the weighted average MCV1 VCR for children 1-6 years of age according to 2 scenarios regarding the VCR in 2023 and 2024. In the first scenario, if the national MCV1 VCR returned to average prepandemic levels (89%) post-2022, the average proportion of children

One to 6 years old who will receive their first dose would be 82% in 2024, 13% below the WHO-recommended VCR of 95% (see Figure, Supplemental Digital Content 3A, http://links.lww.com/INF/F664). In the regional analysis, if the MCV1 VCRs returned to average prepandemic levels post-2022, the proportion of children 1-6 years of age with a missed MCV1 dose in 2024 would be 5%-28% across the different regions (Fig. S4A, http://links.lww.com/INF/F665).

In the second scenario, if the national 2022 MCV1 VCR (81%) continued through 2024, the national VCR in children 1-6 years of age would be 79% in 2024, corresponding to a VCR 16% below the WHO-recommended MCV1 VCR (see Figure, Supplemental Digital Content 3A). In the regional analysis, continuation of the 2022 MCV1 VCRs would result in the regional proportion of children 1-6 years of age who missed their MCV1 dose ranging from 4% to 39% in 2024 (see Figure, Supplemental Digital Content 4B, http://links.lww.com/INF/F665).

MCV2

In the first scenario, if the national MCV2 VCR returned to prepandemic levels post-2022 (91%), 11% of children 6-12 years of age would miss their MCV2 dose, with a national 2024 VCR 1% below the WHO-recommended MCV2 VCR of 90% (see Figure, Supplemental Digital Content 3B, http://links.lww.com/INF/F664). In the regional analysis, if MCV2 VCRs returned to average prepandemic levels, the proportion of children 6-12 years of age with a missed MCV2 dose in 2024 would range from 3% to 17% across regions (see Figure, Supplemental Digital Content 4C, http://links.lww.com/INF/F665).

In the second scenario, if the national 2022 MCV2 VCR continued through 2024 (89%), 12% of children 6-12 years of age would miss their MCV2 dose, and the national 2024 VCR would be 2% below the WHO-recommended MCV2 VCR (see Figure, Supplemental Digital Content 3B, http://links.lww.com/INF/F664). Considering regional VCRs, if 2022 MCV2 VCRs continued through 2024, the regional proportion of children 6-12 years of age who missed their dose would range from 1% to 19% (see Figure, Supplemental Digital Content 4D, http://links.lww.com/INF/F665).

Catch-Up Vaccination Estimations

MCV1

During the pandemic years (2020–2022), 20,856 children missed their MCV1 dose nationally compared to the prepandemic period (2011–2019). Catching up missed MCV1 doses was estimated to take 16-102 months depending on the vaccine catch-up rate (5%–30%) and the MCV1 VCR post-2022 (return to prepandemic VCR versus 2022 VCR; Fig. 4A). Returning to the WHO-recommended MCV1 VCR of 95% was estimated to take 24-158 months depending on the variance in catch-up rate and post-2022 VCR (see Figure, Supplemental Digital Content 5, http://links.lww.com/INF/F666).

FIGURE 4. Number of months required to administer doses missed during the pandemic period (2020–2022) assuming either a return to 2022 or prepandemic average VCR for MCV1 (A) and MCV2 (B).

Most regions would require a monthly catch-up rate of 25%, 16% or 12% for 12-, 18- or 24-month campaigns, respectively, to catch-up missed MCV1 doses while returning to average prepandemic VCRs. The Bor region would not require catch-up, while South Backa, Toplica, Raska and Rasina would require a 31%-73% monthly catch-up rate and were identified as the highest priority regions for MCV1 catch-up vaccination. See Table 1 for regional monthly catch-up rates for a 12-month campaign; catch-up rates for 18- and 24-month campaigns are presented in Figure, Supplemental Digital Content 6, http://links.lww.com/INF/F667.

TABLE 1. Monthly Catch-Up Rates Required to Catch-Up on Missed MCV1 and MCV2 Doses Considering a 12-Month Campaign Based on a Return to Prepandemic VCRs

No catch-up required	Low priority (0%–15% catch-up rate over 12 mo)	Medium priority (16%–30% catch-up rate over 12 mo)	High priority (31%–45% catch-up rates over 12 mo)	Highest priority (>46% catch-up rate over 12 mo)	
MCV1	
 Bor	Central Banat	Branicevo	Belgrade	Rasina	
	Kolubara	Jablanica	Pcinja	Raska	
	North Backa	Macva	Pirot	South Backa	
	North Banat	Morava	Pomorava	Toplica	
	Sumadija	Nisava	West Backa		
		Podunavlje			
		South Banat			
		Srem			
		Zajecar			
		Zlatibor			
MCV2	
 Bor	Branicevo	Pcinja	Belgrade		
Jablanica	Central Banat	Pirot			
Kolubara	Macva	Pomorava			
Nisava	Morava	Rasina			
Zlatibor	North Backa	Srem			
	North Banat	Sumadija			
	Podunavlje				
	Raska				
	South Backa				
	South Banat				
	Toplica				
	West Backa				
	Zajecar				

MCV2

Between 2020 and 2022, 8760 children missed their MCV2 dose nationally compared to the prepandemic period. Catching up missed MCV2 doses would require 6-37 months depending on the catch-up rate (5%–30%) and the MCV2 VCR after 2022 (Fig. 4B).

Across the different regions, to catch up missed MCV2 doses while returning to average prepandemic VCRs for each region, most regions would require a monthly catch-up rate of 14%, 9% or 7% for 12-, 18- or 24-month campaigns, respectively. Five regions that did not miss MCV2 doses would not require catch-up (Bor, Jablanica, Kolubara, Nisava and Zlatibor), while the city of Belgrade would require a 19%-37% catch-up rate. The city of Belgrade was identified as the highest priority for MCV2 catch-up vaccination, followed by Pcinja, Pirot, Pomorava, Rasina, Srem and Sumadija. Table 1 presents catch-up rates for a 12-month campaign, and Figure, Supplemental Digital Content 5, http://links.lww.com/INF/F666, presents catch-up rates for 18- and 24-month campaigns.

DISCUSSION

We quantified the impact of the COVID-19 pandemic on pediatric measles vaccination in Serbia and the need for recovery efforts. Our study showed that during the pandemic, national yearly MCV1 and MCV2 VCRs, which were already below the WHO-recommended threshold in the prepandemic period, decreased to 81% and 89%, respectively. It was estimated that without catch-up vaccination, 18% of children 1-6 years of age and 11% of children 6-12 years of age would have missed their MCV1 and MCV2 doses, respectively, by 2024, assuming a return to average prepandemic national VCRs post-2022. To catch up missed doses under this scenario, most regions would require monthly catch-up rates of 25%, 16% or 12% for MCV1 and 14%, 9% or 7% for MCV2, considering 12-, 18- or 24-month campaigns, respectively. Our study also highlighted the impact of the pandemic across regions in Serbia. South Backa, Toplica, Raska and Rasina were identified as the highest priority regions for MCV1 catch-up, while the city of Belgrade was identified as a high priority for MCV2 catch-up. Notably, while MCV1 VCRs decreased in all regions except Bor, Pcinja and North Banat, MCV2 VCRs increased in ten regions (Kolubara, Raska, Nisava and Jablanica, among others). This may be due to school attendance requirements, along with parents and healthcare professionals delaying vaccination until children are walking and talking.

Globally, several other studies have reported a decrease in pediatric vaccination coverage, including for measles, and have highlighted the need for increased vaccination efforts.14,20,21 This is particularly concerning considering the highly infectious nature of measles.22 In Serbia, a catch-up response was initiated in 2023 although the catch-up rates remain unclear.23 First dose of vaccination is critical, as it provides 93% effectiveness against measles; hence, recovery efforts should prioritize increasing MCV1 VCRs across regions.24 Given regional variation in VCRs for both MCV1 and MCV2, ongoing and future catch-up programs must be tailored to local conditions and prioritized based on the proportion of doses missed in each region. In particular, the data indicate that the pandemic has had a disproportionate impact on immunization in larger, urban centers of Serbia.11 This was also observed in our study, with the 2 most populous regions (the city of Belgrade and South Backa) seeing significant decreases in MCV VCRs. Prioritization of regions most impacted by the pandemic will allow for strategic distribution of resources during recovery efforts. Regional efforts should ensure that mandatory immunization for new birth cohorts continues on schedule alongside catch-up programs.

Catch-up efforts should also consider the resources required to address potential barriers to improving VCRs, such as vaccine hesitancy. Persistently lower VCRs due to vaccine hesitancy have been observed in regions such as South Backa.25,26 Catch-up programs should, thus, consider the importance of improving the public’s knowledge of MCV and its role in preventing measles outbreaks, as such communication has been shown to improve vaccine acceptance.27

This study had some limitations. We used a simple model that estimated VCRs using a ratio for planned and administered vaccines, which may already incorporate catch-up efforts or additional vaccination for unvaccinated children. As such, we may have overestimated the need for catch-up vaccination. Our calculation also evaluated the number of months necessary to catch up missed doses, assuming that the same number of doses would be administered each month, without considering fluctuations in the ability to deliver and receive vaccines. That is, the catch-up rates do not consider implementation challenges that derive from economic, logistical, societal and political factors. Additionally, understanding the factors driving high VCRs in some regions and low VCRs in others may be helpful to standardize national best practices. Another limitation is that large emigrant or immigrant flows were not considered in the cohort VCRs; large flows may bias the estimated VCR. However, recent European data have shown that the number of migrants and tourists arriving in countries is not a significant factor driving increased measles cases, with reduced VCR being the significant driver.28

Overall, this study provides information on the impact of the COVID-19 pandemic on MCV VCRs and highlights the proportion of children who missed MCV doses in Serbia. Our regional analyses are critical to understand where resources should be targeted and what outreach programs are required to allow for strategic vaccination programs. Our study also provides estimated monthly catch-up rates with targeted approaches by region. The catch-up rates required to bridge the gap incurred during the pandemic mean that postpandemic MCV dose administration per month will need to be higher than the prepandemic rates, particularly in the most impacted regions. It is vital that the VCRs in Serbia continue to be monitored to avoid measles cases and outbreaks, assess the effectiveness of MCV catch-up programs and target regions most in need following the pandemic.

ACKNOWLEDGMENTS

The authors thank Adelphi Values PROVE for their support in the development of this article.

Supplementary Material

This study was supported by Merck Sharp & Dohme LLC, a subsidiary of Merck & Co., Inc., Rahway, NJ (MSD).

C.C., C.B., M.P., T.P., B.L. and J.M. are employees of MSD subsidiaries of Merck & Co., Inc., Rahway, NJ. G.L. and M.K. are employees of the Institute of Public Health of Serbia “Dr Milan Jovanovic Batut,” Belgrade, Serbia, and received funding from MSD to attend the conference European Society for Pediatric Infectious Diseases (ESPID) in May 2023.

Supplemental digital content is available for this article. Direct URL citations appear in the printed text and are provided in the HTML and PDF versions of this article on the journal’s website (www.pidj.com).
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