The evolution of Tanzania´s poliomyelitis surveillance system since the introduction of the expanded program on immunization in 1975

¹WHO Inter-Country Support Team office for East and Southern Africa, Harare, Zimbabwe, ²Ministry of Health, Community Development, Gender, Elderly and Children, Dodoma, Tanzania, ³Ministry of Health Zanzibar, Mnazi Mmoja-Zanzibar, Tanzania, ⁴WHO Country Office, Tanzania, Dar es Salaam, Tanzania

^&Corresponding author
Daudi Manyanga, WHO Inter-Country Support Team office for East and Southern Africa, Harare, Zimbabwe

Introduction    

Tanzania started her Expanded Programme on Immunization (EPI) as a project to deliver vaccines against diphtheria, pertussis, tetanus, measles, poliomyelitis, smallpox, and tuberculosis in 1975 which is one year following the World Health Organization (WHO) resolution WHA27.57 [1,2]. The WHO on the Twenty- seventh World Health Assembly (WHA) sat on 23 May 1974, recommended to develop or maintain immunization and surveillance programs against some or all of the following diseases: diphtheria, pertussis, tetanus, measles, poliomyelitis, tuberculosis, smallpox, and others, where applicable, to it is member states [3]. In the first five years of EPI introduction in Tanzania (1975-1980), vaccines, preventable diseases especially measles were the second cause of hospital admission and contributed 15% of the total hospital under 15 years of children's deaths of which 70% were from under two years of age [1]. Globally, vaccination coverage under the EPI remained suboptimal in that period, and while outbreaks have occurred in developed countries, they were significantly more pronounced in developing or low-income nations [4].

In 1980, a comprehensive evaluation conducted by the Ministry of Health, the World Health Organization (WHO), the United Nations International Children's Emergency Fund (UNICEF), and the Danish International Development Agency (DANIDA) led to substantial investments in the Expanded Program on Immunization [4,5]. These investments were directed towards the implementation, training, and development of cold chain infrastructure, commencing in 1982 and continuing through 1986. In January 1986, the Government of Tanzania designated the EPI as a priority program [6]. Although vaccine-preventable disease surveillance was initially unreliable, a record review indicated a consistent and rapid decline in the incidence of both measles and pertussis in reporting units since 1981. This decline may be attributed to the enhancements in the EPI, particularly in the vaccine distribution system.

Studies reported that following EPI introduction, the vaccination coverage in Tanzania was below 55% by 1980 and reached only 67% by 1985 using DTP3 (representing OPV3) as a proxy vaccination indicator. From 1985, the country implemented universal childhood immunization which raised coverage from 67% (1985) to 85% (1988) [7,8]. Poliomyelitis (regarded as acute poliomyelitis) was among the major cause of disease admissions in hospitals in Tanzania before the implementation of universal childhood immunization by 1988. For example, Kilimanjaro Christian Medical Centre reported 19 acute polio cases admissions in 1983 [9]. Prior to the introduction of the Expanded Program on Immunization (EPI), poliomyelitis was among the prevalent diseases in mainland Tanzania, previously referred to as Tanganyika. In 1953, the World Health Organization (WHO) reported that out of 2,170 polio cases, Tanganyika accounted for 136 cases and 22 deaths, indicating an increase from 63 cases and 6 deaths in 1949 [10,11].

The Pan America Health Organization (PAHO) initiated the rally for the global polio eradication in 1985 based on the scientific fact that the infectious agent for the disease, poliovirus has no animal reservoir, cannot stay long in the environment, confer long-term immunity, and the availability of effective vaccines [12]. This finally resulted in the WHO resolution in 1988 to eradicate polio by the year 2000 [13]. The main challenges by that time were the suboptimal vaccination coverage, polio eradication was not a priority to some member states, lack of effective surveillance and immunization programs, low immunogenicity of OPV (tropical countries including Tanzania), and inadequate financial commitments [12].

Regional of America eliminated polio in 1994 nevertheless, the disease remained to be reported from other countries including Tanzania. In that year, the country had a well-established routine immunization program with the OPV3 vaccination coverage of 82% [14]. Furthermore, all regions and districts in Tanzania received training in the methods of AFP case investigation and established reverse cold chain to handle stool specimens. Nonetheless, only 21.3% (25) of the 117 districts had a well-established AFP case-based surveillance by 1995 [15]. In this regard, the AFP surveillance was neither sensitive nor specific for the detection, investigation, and reporting of the polio cases. Additionally, the first OPV mass vaccination campaign for children under five years was conducted in 1995 as a vaccination response to wild poliovirus outbreak in the Mbeya region. The country conducted National immunization days in 1996, 1997, and 1998 as part of implementing a polio eradication strategy [15].

In 1995 the WHO set standards for AFP surveillance to be reached by the member states of which was the detection of at least 1 case per 100,000 population age less than 15 years, collection of 2 stool samples to at least 80% of the reported AFP cases within 2 weeks of the onset of paralysis, transport 90% of stool samples to the respective laboratory in a satisfactory condition and isolate at least 10% nonpolio enterovirus in the collected stool samples [14]. Confirmed polio cases in that time were based on the clinically report of the reported AFP with any of the following: laboratory-confirmed WPV, residual paralysis at 60 days, death, or loss to follow-up at 60 days [14]. Surveillance for AFP and wild poliovirus was used and will continue to be used to monitor the progress in interrupting poliovirus transmission and document the absence of wild poliovirus from the country and achieve polio eradication. The last case of WPV in Tanzania was reported in 1996 with the date of onset of 31/7/1996. In 1997 the Non-polio AFP rate for the country was still 0.11 per 100,000 population aged less than 15 years. In this regard, Tanzania was referred to as a non-reporting country rather than a county with zero cases [16].

Tanzania significantly enhanced the sensitivity of its national surveillance system, increasing the non-polio acute flaccid paralysis (AFP) rate from 0.11 per 100,000 population under 15 years of age in 1997 to 1 per 100,000. Similarly, the percentage of stool adequacy improved from 65% in 1997 to 87% in 1999. Additionally, the country transitioned from the clinical classification of reported AFP cases to virological classification. The fluctuating performance of AFP surveillance over the years necessitated a tailored solution to improve and sustain surveillance efforts, leading to the establishment of the National Stop Transmission of Polio (NSTOP) teams. These teams were modeled after the Centers for Disease Control and Prevention (CDC) Polio Stop Transmission of Polio (STOP) teams. The CDC, leveraging the platform of the Global Polio Eradication Initiative (GPEI) and with support from the World Health Organization (WHO), UNICEF, Rotary International (RI), and the Bill and Melinda Gates Foundation (BMGF), established the STOP program in 1999 [17]. The STOP program primarily recruits and trains volunteer international public health professionals for the strengthened immunization and surveillance system to countries following their deployment.

In 2012, Tanzania initiated the National Stop Transmission of Polio (NSTOP) program to intensify the active search for vaccine-preventable diseases. The program involves the selection, training, and deployment of national public health professionals to underperforming districts for two weeks. The primary NSTOP personnel were recruited from well-performing districts and comprised public health professionals working in the public sector. Their deployment followed three days of training on acute flaccid paralysis (AFP), measles, and other vaccine-preventable diseases (VPDs) field surveillance guidelines. Real-time online monitoring is utilized to track their supportive supervision and active search visits. Upon completion of the two-week assignment, the deployed NSTOP teams submit both technical and financial reports to the national surveillance secretariat.

The initiative successfully strengthened polio surveillance, enabling Tanzania to achieve polio-free certification in 2015, a positive milestone contributing to the Africa regional certification in August 2020. Furthermore, Tanzania introduced environmental surveillance in 2019, which, in addition to the sensitive AFP surveillance, ensures the country's polio surveillance system remains robust. Currently, this system detects polio outbreaks earlier than AFP surveillance alone. We recognized the lack of comprehensive documentation detailing the evolution of the Expanded Program on Immunization (EPI) and polio surveillance in Tanzania. Consequently, we aimed to publish and provide thorough documentation of the milestones and long-term progress of the EPI program, including polio surveillance, over the 50 years since the program's inception.

Methods     

We conducted a descriptive, qualitative, and quantitative study utilizing secondary data analysis of the acute flaccid paralysis (AFP) surveillance database from 1980 to 2024. The data were sourced from various records shared by the Ministry of Health with the World Health Organization (WHO). These sources included the weekly field AFP case-based surveillance database, the WHO/UNICEF joint reporting forms, and program review reports. Most of these data are available in the public domain on WHO websites. To assess surveillance performance over this period in Tanzania, we employed the WHO standard AFP surveillance performance indicators [18]. This comprehensive approach allowed us to evaluate the effectiveness and responsiveness of the surveillance system, providing valuable insights into the trends and outcomes of polio eradication efforts in the country.

We also noted that although the Expanded Program on Immunization (EPI) was initiated in 1975 in Tanzania, services were not available in all regions until 1980. This expansion followed a joint evaluation by the Ministry of Health, WHO, UNICEF, and DANIDA, which included training, installation of cold chain facilities, and implementation efforts from 1982 to 1986. Consequently, data collected from 1975 to 1980 were derived from surveys and publication reviews and were used for qualitative analysis and discussion. In contrast, secondary data from 1980 to 2024 can be retrieved from WHO databases.

For this review, we categorized the evolution of the polio surveillance system in Tanzania into four distinct periods: the aggregated polio surveillance phase (1980-1994), the clinical classification period (1995-2000), the polio-free pre-certification period (2001-2015), and the polio-free post-certification period (2016-2024). The indicators used in this study include vaccination coverage for the third dose of oral polio vaccine (OPV3) and the third dose of Diphtheria, Tetanus, and Pertussis (DTPw) for infants, based on the WHO/UNICEF Estimates for National Immunization Coverage (WUENIC). Additional indicators include the number of reported polioviruses (wild poliovirus or vaccine-derived variants) and their sources, the number of reported acute flaccid paralysis (AFP) cases, the non-polio acute flaccid paralysis (NP-AFP) rate, the number of polio-compatible cases, and the percentage of stool samples that are adequate. For environmental surveillance (ES), the indicators include geographical coverage (proportion of regions with ES sites) and the proportion of ES sites with an enterovirus isolation rate of 50% or more. To assess the sensitivity of the AFP surveillance system in Tanzania, we employed the non-polio AFP (NPAFP) rate. This indicator reflects the number of cases discarded as NPAFP in children under 15 years of age per 100,000 population of the same age group annually. Additionally, we used the adequate stool specimen collection indicator, which measures the percentage of AFP cases with two stool specimens collected at least 24 hours apart, both within 14 days of paralysis onset, and the arrival of these specimens in good condition at a WHO-accredited laboratory. Data were analyzed using the CDC's public domain software package EPI Info version 3.5.4, supplemented by Microsoft Excel and SPSS version 22 for inferential statistics. For numerical values, proportions, and percentages, trend analysis was conducted on reported polioviruses (wild poliovirus or vaccine-derived variants) to observe changes and triangulate with vaccination coverage trends.

Results     

In our study, a total of 1,576 wild polioviruses were reported during the review period from 1980 to 1994, with the majority of cases (70.3%) occurring between 1985 and 1987 (Table 1). This period is nearly a decade after the initiation of the Expanded Program on Immunization (EPI) in Tanzania. However, vaccination coverage remained relatively low, with OPV3 coverage below 50% and DTPw3 coverage below 60%. The mean vaccination coverage was 67.6% for OPV3 and 73% for DTPw3, while the median coverage was 71% for OPV3 and 77% for DTPw3. The lowest OPV3 coverage during this period was 46% in 1981, with the highest being 83% in 1994. For DTPw3, the lowest coverage was 50% in 1982, and the highest was 85% in 1988. The high number of reported polio cases may be underestimated due to the limited sensitivity of the surveillance system at that time. Nonetheless, studies indicate a dramatic decline in other vaccine-preventable diseases (VPDs) such as measles and diphtheria, suggesting that without the immunization program, the incidence of polio cases could have been ten to hundreds of times higher.

During the AFP case-based surveillance phase (1995-2000), only four wild polioviruses were isolated from AFP cases, specifically in 1995 (3) and 1996 (1). However, a total of 414 AFP cases were reported, of which 182 (43.9%) were ultimately classified as polio compatible. In terms of vaccination coverage, the average OPV3 coverage was 75.8%, while DTPw3 coverage averaged 79.3%, with both antigens having a median coverage of 79%. The lowest OPV3 coverage was 64% in 2000, while the highest was 80% in 1996. The NP-AFP rate reached 1.5 per 100,000 population under 15 years of age in 2000, with 79% stool adequacy (Table 2).

During the polio-free pre-certification period from 2001 to 2014, no wild poliovirus was isolated. A total of 6,192 AFP cases were reported, of which 137 (2.2%) were ultimately classified as polio compatible. The NP-AFP rate increased from 0.9 per 100,000 population under 15 years of age in 2003 and 2004 to 4.3 in 2015. The percentage of stool adequacy improved from 70% in 2003 to 97% in 2015. The mean OPV3 coverage was 89.9%, while the mean DTPw3 coverage was 90.5%. The median vaccination coverage was 91% for OPV3 and 90% for DTPw3. In terms of the impact of the deployment of the NSTOP program in 2012, we observed an increase in the reported AFP cases by 177, although this difference was not statistically significant (p-value = 0.314). Additionally, there was a non-significant increase in the percentage of stool adequacy from 92% in 2011 to 95% in 2012 (p-value = 0.052, 95% CI: -0.038 to 6.345). Furthermore, from that year onwards, the national average Non-Polio AFP (NP-AFP) rate increased over the years, surpassing the certification level requirement of 2 per 100,000 population aged less than 15 years. The NP-AFP rate in 2015 was 4.3, which represents a significant variation of 2.4 from the rate in 2011 (p-value = 0.03, 95% CI: 0.225 to 4.287). Additionally, the percentage of stool adequacy increased from 92% in 2011 to 97% in 2015, indicating a 5% change, although this was not statistically significant (Table 3).

In the post-polio-free certification phase (2016-2024), no wild poliovirus was isolated. However, fifteen vaccine-derived poliovirus type 2 (cVDPV2) variants were reported: two from AFP cases, seven from environmental surveillance (ES), and six from healthy individuals. During this period, a total of 9,623 AFP cases were reported, of which 109 (1.1%) were ultimately classified as polio-compatible (Table 4). Vaccination coverage for OPV3 and DTPw3 remained high (over 80%) throughout the period, except in 2020 (OPV3: 61%) and 2021 (OPV3: 66%). The NP-AFP rate consistently exceeded 3, with the lowest rates in 2019 and 2021 (3.5), and peaking at 6 in 2023. The percentage of stool adequacy remained high (over 90%), with the lowest being 93% in 2021. Environmental surveillance (ES) was introduced in 2019 in only one region (3%) of Tanzania's 31 regions, expanding to twelve regions (39%) by 2023. From 2019 to 2024, the enterovirus isolation rate remained robust, with all ES sites achieving an EV isolation rate of over 50%, except in 2023, when 87.5% of the sites had an EV isolation rate of over 50%.

From the analysis of the WHO/UNICEF joint reporting form data spanning 1980 to 2023, Tanzania reported a total of 671,569 measles cases, 26,020 pertussis cases, 3,676 neonatal tetanus cases, 174 diphtheria cases, and 47 congenital rubella syndrome cases. When the data was segmented into five-year intervals, it was observed that the period from 1980 to 1984 accounted for 52.6% of the total reported measles cases (Chart 1), 39.9% of the total pertussis cases, 31.4% of the overall neonatal tetanus cases, and 100% of the total diphtheria cases. This significant concentration of cases in the early 1980s highlights the critical immunization and overall public health challenges faced during that time. Recognizing the interconnected nature of polio and measles surveillance, we observed notable trends in vaccination coverage over the period from 1980 to 2023. Using data from the WHO/UNICEF Estimates of National Immunization Coverage (WUENIC), the mean coverage for the first dose of the measles-containing vaccine (MCV1) increased from 52.4% in the initial five-year segment to 86.8% in the most recent five-year segment. Similarly, the coverage for the third dose of the diphtheria-tetanus-pertussis vaccine (DTPw) rose from 52.4% to 90.7%, and the coverage for two or more doses of tetanus toxoid (TT2+) administered to pregnant women increased from 33.8% to 64.9% over the same periods. These improvements reflect significant progress in vaccination efforts and public health initiatives (Figure 1).

Discussion     

In our study, we observed that prior to the introduction of the Expanded Program on Immunization (EPI) and polio surveillance, poliomyelitis was a major cause of disability and death in Tanzania, with limited documentation available. During the aggregate polio surveillance phase, over 1,576 poliomyelitis cases were reported. As the geographical and vaccination coverage of the EPI program increased, a dramatic decline in cases was noted, regardless of the initiation of case-based surveillance.

Despite vaccination coverage being less than 80% during the initial phase from 1980 to 1994, we noted a significant decline in polio cases by the end of this period. In 1996, the impact of vaccination campaigns in raising population immunity and halting transmission was again evident. The first nationwide mass vaccination campaign in 1996 also provided an opportunity to enhance the sensitivity and quality of polio surveillance. Consequently, even with OPV3 coverage at 64% and more sensitive case-based surveillance in 2000, no wild poliovirus was detected. This was lower than the 80% coverage in 1996 when wild poliovirus cases were present. The national OPV vaccination campaigns from 1996 to 1998 significantly boosted population immunity against polio. The deployment of the National Stop Transmission of Polio (NSTOP) program in 2012 significantly improved the surveillance system in Tanzania. This enhancement facilitated the country in meeting the certification requirements, which were subsequently accepted by the African Regional Certification Commission in December 2015. The initiative is cost-effective and has long-term benefits, making it a viable model for adoption by other countries with similar contexts.

In the post-polio-free certification phase, we observed the maintenance of sensitive surveillance systems at both national and subnational levels, along with the implementation of a highly sensitive supplementary environmental surveillance (ES) system. The model of the polio surveillance system in Tanzania is crucial for informing and integrating other vaccine-preventable disease (VPD) surveillance systems. The emergence and importation of vaccine-derived poliovirus variants (cVDPV) continue to highlight the sensitivity of the polio surveillance system, despite declining funds and other competing health priorities. However, the global health environment in 2025 is complex, making it difficult to predict the future sustainability of the significant achievements and milestones reached not only in Tanzania but also in other low- and middle-income developing countries [19,20].

Our study revealed a significant decline in measles cases, from 102,127 cases in 1982 to zero cases in 2020. This reduction can be attributed to high vaccination coverage and COVID-19 prevention measures. Despite maintaining high vaccination coverage in 2023 (91% according to WUENIC estimates), the number of measles cases rose to 2,887, which is comparable to figures from over a decade ago, specifically 2008. This trend underscores the importance of continuously triangulating vaccination coverage with case data to enhance decision-making processes. Consequently, prioritizing and adequately funding vaccine-preventable disease (VPD) surveillance is crucial for effective public health management.

Conclusion     

There has been an overall improvement in the sensitivity and quality of the polio surveillance system in Tanzania since the initiation of the Expanded Program on Immunization (EPI). The EPI program has been instrumental in demonstrating the impact of vaccinations through a sensitive surveillance system capable of indicating changes in disease patterns. The evolution of the polio surveillance system, from aggregate surveillance to case-based laboratory surveillance, should serve as a model for strengthening other vaccine-preventable disease (VPD) surveillance systems to control, eliminate, and eradicating these diseases in the future. The environmental surveillance (ES) system designed for polio should be expanded, optimized, and utilized to detect other VPDs. The case of Tanzania illustrates how the World Health Organization (WHO) not only impacted public health by initiating the EPI program but also established a robust mechanism to evaluate the impact of the immunization program through an effective surveillance system.

Competing interests     

The authors declare no competing interests.

Authors' contributions     

Daudi Manyanga and Honest Nyaki conceptualized and designed the study, and constructed the background information, study analysis, interpretation, discussion, critical review and organization of, the entire manuscript for publication. Charles Byabamazima, Georgina Joachim, Abdul Ameir Saleh, and Boniface Makelemo participated in the study analysis, interpretation, discussion, and critical review of the manuscript for publication. Sarah Wanyoike participated in the study analysis, interpretation, discussion, and review of the manuscript. All the authors read and approved the final version of this manuscript.

Acknowledgments     

We acknowledge efforts made by all Surveillance Officers and Data Managers from Ministries of Health, WHO and other partners in the ESA countries for their untired efforts till polio will be eradicated.

Tables and figure     

Table 1: summary of the vaccination coverage for OPV3 and DTP3, along with reported wild polioviruses in Tanzania from 1980 to 1994

Table 2: distribution of OPV3 and DTP3 vaccination coverage, reported AFP cases, wild polioviruses, polio-compatible cases, and AFP surveillance core indicators by year in Tanzania from 1996 to 2000

Table 3: distribution of OPV3 and DTP3 vaccination coverage, reported AFP cases, wild polioviruses, polio-compatible cases, and AFP surveillance core indicators by year in Tanzania from 2001 to 2015

Table 4: vaccination coverage of OPV3 and DTP3, reported AFP cases, wild polioviruses, polio-compatible cases, core indicators for AFP surveillance, and environmental surveillance performances by year in Tanzania from 2016 to 2024

Figure 1: trends in reported measles cases and WUENIC MCV1 vaccination rates in Tanzania, 1980-2023

References