Surveillance of adverse events following the introduction of inactivated poliovirus vaccine made from Sabin strains (sIPV) to the Chinese EPI and a comparison with adverse events following inactivated poliovirus vaccine made from wild strains (wIPV) in Jiangsu, China

ABSTRACT

One dose of inactivated poliovirus vaccine (IPV) was introduced into the Chinese Expanded Program on Immunization (EPI) in 2016. IPV made from Sabin strains (sIPV) was newly licensed in China and its safety has been concerned. This study aimed to evaluate the safety of sIPV and provide a comparison with conventional IPV made from wild strains (wIPV). We collected all IPV-related AEFI reports in Jiangsu from the Chinese National Adverse Events Following Immunization Information System (CNAEFIS) for 2016–2019. We obtained the administered doses of IPV from the Jiangsu provincial Electronic Immunization Registries System (JSEIRS). The AEFI reporting rates per 100,000 doses of vaccine administered were compared for sIPV and wIPV. A total of 699 sIPV and 908 wIPV AEFI cases were collected by CNAEFIS in Jiangsu during 2016–2019. The overall AEFI reporting rates were 53.02 per 100,000 doses and 41.25 per 100,000 doses for sIPV and wIPV, respectively (P < .001). For both sIPV and wIPV, the AEFIs were mainly classified as common adverse reactions. The reporting rate of common adverse reactions was higher for sIPV than for wIPV (P < .001). The most frequently reported symptoms/signs were fever, persistent crying, injection site erythema/swelling, rash, and injection site induration. Only 1.14% of sIPV-associated and 2.31% of wIPV-associated AEFI cases were diagnosed as serious. No difference in reporting rate was observed for serious AEFIs (P = .272). sIPV has a favorable safety profile, although it exhibits a slightly higher reporting rate of common adverse reactions than wIPV.

KEYWORDS:

• Vaccine safety
• adverse events following immunization
• inactivated poliovirus vaccines
• post-marketing surveillance
• expanded program on immunization

1. Introduction

The Polio Eradication and Endgame Strategic Plan 2013–2018 recommended the globally synchronized withdrawal of OPV type 2 in 2016 and a switch from trivalent OPV (tOPV), containing all 3 types of poliovirus, to the bivalent OPV (bOPV), containing only types 1 and 3.¹ The WHO Strategic Advisory Group of Experts (SAGE) also recommended that at least one dose of inactivated poliovirus vaccine (IPV) should be included in the routine immunization schedules in OPV-using countries to protect against possible outbreaks of poliovirus type 2.^1,2 According to the WHO recommendations, China switched from using tOPV to using bOPV on May 1, 2016,³ a switch that was synchronized with 154 other countries and territories using OPV in their immunization programs.⁴ At the same time, China introduced one dose of IPV into their Expanded Program on Immunization (EPI), with a new routine schedule of one IPV dose given at 2 months of age, followed by a bOPV dose at 3 months, 4 months, and 48 months of age.^5,6

Two types of IPV vaccines were included in the Chinese EPI, namely, IPV made from Sabin strains (sIPV) and IPV made from wild strains (wIPV).⁶ wIPV has been available in China as a private sector vaccine since 2009. sIPV, produced by domestic manufacturers in China, was licensed in 2015 as the first standalone sIPV in the world. wIPV or wIPV-containing vaccines have been widely used in the USA and in other countries, and they have proven to be safe and effective.^7-9 However, due to the high biosafety risks during production, the limited production capacity, and the high cost, wIPV cannot be widely used in many developing countries.^10,11 In view of the above factors, WHO encourages the research and development of new IPVs that use less virulent strains, such as sIPV.¹² sIPV carries a lower biosafety risk and is both an effective and economical option for low-income and middle-income countries such as China.^13,14 sIPV is now licensed and used only in China and Japan.¹⁵ Clinical trials have been conducted to evaluate the safety and immunogenicity of the sIPV vaccine and the sIPV-OPV sequential immunization schedule.^16-20 However, clinical vaccine trials usually involve a limited number of study subjects and are rarely powered sufficiently to detect rare adverse events following immunization (AEFI). Therefore, post-marketing surveillance of AEFIs is particularly important for the safety assessment of new vaccines when they are routinely used in the general population.^8,21 Few studies of the post-licensure AEFI surveillance of sIPV were published after pilot projects launched in China in 2015 (replaced the first dose of tOPV with sIPV before IPV was included in the EPI).^22-24 However, to the best of our knowledge, limited data are available regarding the safety of sIPV after its inclusion in the Chinese EPI and, consequently, its large-scale use. Continuous monitoring based on a large enough population helps to capture rare AEFI and comprehensively evaluate the safety of a new vaccine.

This study aimed to comprehensively evaluate the safety of sIPV. We summarized all sIPV AEFIs and AEFI reporting rates from passive surveillance data collected in the first 4 y (2016 to 2019) after the inclusion of sIPV in the EPI in Jiangsu, China. These data were compared to those of wIPV over the same period. This study may provide a reference for the wide-spread use of sIPV in developing countries at the endgame of polio eradication.

2. Materials and methods

2.1. Vaccination schedules of polio vaccines in Jiangsu, China

Jiangsu is a province with a population of approximately 80 million people and is located on the eastern coast of mainland China. Since April 1^st, 2016, one dose of IPV has been included in Jiangsu’s EPI to be given at 2 months of age. On September 1^st, 2019, two doses of IPV were included in Jiangsu’s EPI to be given at 2 months and 3 months of age, respectively. Both sIPV and wIPV were included in Jiangsu’s EPI and purchased by the government. For an individual child, there is no specific rule to decide which IPV vaccine type should be given. It just depends on what IPV vaccine type is available in the vaccination clinic.

2.2. Administered doses of IPV

The doses of sIPV and wIPV (standalone wIPV only, combination vaccines were not included) administered between 2016 and 2019 (inclusive) in Jiangsu were obtained from Jiangsu Provincial Electronic Immunization Registries System (JSEIRS). JSEIRS was built and maintained by the Jiangsu Provincial Center for Disease Control and Prevention (JSCDC) to collect and manage vaccination information, vaccine information, vaccination clinic information, and cold chain temperature information. All of the identified vaccines should be used in vaccination clinics, and every recipient’s vaccination information is submitted to JSEIRS by vaccination nurses.

2.3. AEFI collection

AEFIs reported following vaccination with sIPV or wIPV administered between 2016 and 2019 (inclusive) in Jiangsu were extracted from the China National AEFI Information System (CNAEFIS) on March 1^st, 2020. The CNAEFIS is an official immunization safety surveillance system that was established by the Chinese CDC based on World Health Organization (WHO) guidelines. Jiangsu began to use CNAEFIS for long-term and systematic surveillance in 2007. The CNAEFIS includes submitted AEFI reports from passive surveillance in Jiangsu.

2.4. AEFI reporting and investigation

China’s national AEFI guidelines were released in 2010,²⁵ and Jiangsu Province carried out AEFI monitoring and reporting according to this guidance. An AEFI case is defined as a reaction or an event occurring after vaccine administration that is suspected to be related to the vaccination. Healthcare facilities, vaccination clinics, CDC units at the provincial, prefectural, and county levels, adverse drug reaction monitoring agencies (ADRs), and vaccine manufacturers are required by law to report AEFI cases. Members of the public, including guardians (parents), can notify any of the above-authorized reporters to report an AEFI case. In Jiangsu, after identifying AEFI cases, all the above-authorized reporters should report it to the vaccination clinic or the county-level CDC where the vaccination clinic is located. The vaccination clinic or the county-level CDC then completes an “AEFI Case Reporting Card” and submits the data to CNAEFIS. Once the information is entered, it can be viewed by all administrative levels of the CDCs and ADRs.

An AEFI is any adverse medical occurrence that follows immunization but which does not necessarily have a causal relationship with the vaccine administration. According to the cause-specific categorization of AEFIs, revised by the Council for International Organizations of Medical Sciences (CIOMS) and WHO in 2012,²⁶ AEFIs are divided into five types: vaccine product-related reactions (including common adverse reactions and rare adverse reactions), vaccine quality defect-related reactions, immunization error-related reactions, immunization anxiety-related reactions, and coincidental events. The investigation is required for all AEFIs, except for the common adverse reactions with a clear diagnosis (e.g., fever, erythema, swelling, and induration at the injection site). AEFI cases are investigated by the county-level CDC. In the case of deaths, serious AEFIs, AEFI clusters, and AEFIs of significant public concern that are suspected to be related to vaccination, prefectural or provincial expert committees (comprising pediatricians, physicians, epidemiologists, pharmacists, vaccine researchers, etc.) are responsible for further investigation and causality assessment.

2.5. Serious AEFIs

A serious AEFI is defined as an event that results in death, is life-threatening, requires inpatient hospitalization or prolongation of existing hospitalization, results in persistent or significant disability/incapacity, is a congenital anomaly/birth defect, or requires intervention to prevent permanent impairment or damage.²⁶ According to China’s national AEFI guidelines, serious AEFIs include but are not limited to allergic shock, allergic laryngeal edema, allergic purpura, thrombocytopenic purpura, localized allergic necrotic reaction (Arthus reaction), febrile convulsion, epilepsy, brachial neuritis, polyneuritis, Guillain–Barre syndrome, encephalopathy, encephalitis and meningitis, syncope, toxic shock syndrome, and systemic purulent infection.²⁵

2.6. Data analysis

We analyzed AEFI reports submitted throughout 2016–2019 for subjects vaccinated with sIPV or wIPV. In CNAEFIS, a maximum of three suspected vaccines can be reported at the same time in one AEFI record. The vaccine that most likely caused the event is listed as the first suspected vaccine in the record. In this analysis, all records that listed sIPV or wIPV as the first, second, or third suspected vaccine were included. Each AEFI record may list several symptoms, signs, and/or diagnoses. All symptoms and signs were included when the AEFI was classified as a “common adverse reaction”; only the main symptom or the most serious diagnosis was included when the AEFI was classified as a “rare adverse reaction”.

The administered doses of sIPV and wIPV were used as the denominator to calculate the reporting rates. The reporting rates of AEFIs (per 100,000 administered doses) were analyzed by cause-specific categorization and by symptoms/signs/diagnosis categorization.

The Pearson chi-square test or Fisher’s exact test was used to examine differences between sIPV and wIPV. If at least one theoretical frequency of the cross-table was lower than 5, Fisher’s exact test was used, otherwise, the Pearson chi-square method was used. Hypothesis testing was two-sided, and P-values <0.05 were considered to be statistically significant. All statistical analyses were performed using R software, v3.5.3.

3. Results

3.1. AEFIs following sIPV and wIPV administration

Overall, 1,318,255 doses of sIPV and 2,200,956 doses of wIPV were administered to children in Jiangsu throughout 2016–2019. Subsequently, a total of 699 sIPV and 908 wIPV AEFI cases was reported. The overall AEFI reporting rate for sIPV was 53.02 per 100,000 doses, and the annual AEFI reporting rates for sIPV varied from 44.25 to 61.30 per 100,000 doses. The overall AEFI reporting rate for wIPV was 41.25 per 100,000 doses, varying between 37.73 and 45.41 per 100,000 doses over the included timeframe. The overall AEFI reporting rate was slightly higher for sIPV than for wIPV (P < .001) (Table 1).

Table 1. Reporting rates of sIPV and wIPV by cause-specific categorization of AEFI, Jiangsu, China, 2016–2019

AEFI	sIPV									wIPV
	Doses	Common adverse reactions		Rare adverse reactions		Coincidental events		Total		Doses	Common adverse reactions		Rare adverse reactions		Coincidental events		Total
		No.	Reporting rates*	No.	Reporting rates*	No.	Reporting rates*	No.	Reporting rates*		No.	Reporting rates*	No.	Reporting rates*	No.	Reporting rates*	No.	Reporting rates*
2016	58757	25	42.55	1	1.70	0	0.00	26	44.25	766304	314	40.98	29	3.78	5	0.652	348	45.41
2017	223196	104	46.60	10	4.48	3	1.34	117	52.42	636111	198	31.13	31	4.87	11	1.729	240	37.73
2018	608892	265	43.52	26	4.27	3	0.49	294	48.28	270839	95	35.08	7	2.58	2	0.738	104	38.40
2019	427410	246	57.56	14	3.28	2	0.47	262	61.30	527702	189	35.82	23	4.36	4	0.758	216	40.93
2016–2019	1318255	640	48.55	51	3.87	8	0.61	699	53.02	2200956	796	36.17	90	4.09	22	1.000	908	41.25
P value												< .001^†		.752 ^‡		.222^§		< .001**

*Reporting rates: per 100,000 doses administered.

^†Comparison the reporting rate of common adverse reactions between sIPV and wIPV, 2016–2019.

^‡Comparison the reporting rate of rare adverse reactions between sIPV and wIPV, 2016–2019.

^§Comparison the reporting rate of coincidental events between sIPV and wIPV, 2016–2019.

** Comparison the reporting rate of AEFI between sIPV and wIPV, 2016–2019.

For both sIPV and wIPV, the AEFIs were mainly classified as common adverse reactions (91.56%, n = 640; 87.67%, n = 796, respectively), followed by rare adverse reactions (7.30%, n = 51; 9.91%, n = 90, respectively) and coincidental events (1.14%, n = 8; 2.42%, n = 22, respectively). No AEFI reports were classified as vaccine quality defect-related reactions, immunization error-related reactions, or immunization anxiety-related reactions.

The average reporting rate of common adverse reactions was higher for sIPV (48.55 per 100,000 doses) than for wIPV (36.17 per 100,000 doses) (P < .001). There was no significant difference between the average reporting rates of rare adverse reactions for sIPV (3.87 per 100,000 doses) and wIPV (4.09 per 100,000 doses) (P = .752). Similarly, no significant difference was seen between the average reporting rates of coincidental events for sIPV (0.61 per 100,000 doses) and wIPV (1.00 per 100,000 doses) (P = .222).

For both sIPV and wIPV, the top five most frequently reported symptoms and/or signs were fever, persistent crying, injection site erythema and/or swelling, rash, and injection site induration. Fever, persistent crying, and injection site induration were reported at a significantly higher rate following sIPV administration compared to wIPV (P < .05) (Table 2). Conversely, drowsiness was reported at a significantly lower rate following sIPV administration compared to wIPV (P = .025).

Table 2. Symptoms/signs/diagnosis of AEFI after the vaccination of sIPV and wIPV, Jiangsu, China, 2016–2019

AEFIs symptoms and/or signs	sIPV			wIPV			P value
	No.	%^†	Reporting rates*	No.	%^†	Reporting rates*
Fever	432	40.22	32.77	529	36.31	24.04	< .001
Persistent crying	194	18.06	14.72	259	17.78	11.77	.018
Injection site erythema and/or swelling	182	16.95	13.81	255	17.50	11.59	.070
Rash	84	7.82	6.37	136	9.33	6.18	.825
Injection site induration	59	5.49	4.48	60	4.12	2.73	.006
Poor appetite	41	3.82	3.11	56	3.84	2.54	.328
Fatigue	21	1.96	1.59	29	1.99	1.32	.507
Vomiting	18	1.68	1.37	16	1.10	0.73	.062
Drowsiness	9	0.84	0.68	34	2.33	1.54	.025
Pruritus	7	0.65	0.53	13	0.89	0.59	.820
Cough	6	0.56	0.46	9	0.62	0.41	.839
Diarrhea	5	0.47	0.38	20	1.37	0.91	.071
Thrombocytopenia	5	0.47	0.38	13	0.89	0.59	.396
Nausea	3	0.28	0.23	2	0.14	0.09	.370
Pallor	2	0.19	0.15	3	0.21	0.14	1.000
Allergic purpura	2	0.19	0.15	0	0.00	0.00	.140
Myalgia	2	0.19	0.15	1	0.07	0.05	.560
Bronchopneumonia	1	0.09	0.08	6	0.41	0.27	.268
Death	1	0.09	0.08	3	0.21	0.14	1.000
Headache	0	0.00	0.00	2	0.14	0.09	.531
Hyperidrosis	0	0.00	0.00	1	0.07	0.05	1.000
Angioedema	0	0.00	0.00	1	0.07	0.05	1.000
Cyanosis of lips	0	0.00	0.00	1	0.07	0.05	1.000
Epilepsy	0	0.00	0.00	1	0.07	0.05	1.000
Hemolytic anemia	0	0.00	0.00	1	0.07	0.05	1.000
Encephalitis and meningitis	0	0.00	0.00	2	0.14	0.09	.531
Encephalopathy	0	0.00	0.00	1	0.07	0.05	1.000
Sepsis	0	0.00	0.00	1	0.07	0.05	1.000
Thrombocytosis	0	0.00	0.00	1	0.07	0.05	1.000
Pulmonary hypertension	0	0.00	0.00	1	0.07	0.05	1.000

*Reporting rates: per 100,000 doses administered.

^†%: % of all reported AEFIs.

3.2. Serious AEFIs

Of all the AEFI cases, 8 (1.14%) sIPV-associated and 21 (2.31%) wIPV-associated AEFI cases were diagnosed as serious. There was no difference in the overall reporting rate of serious AEFIs between sIPV (0.61 per 100,000 doses) and wIPV (0.95 per 100,000 doses) (P = .272). For both sIPV and wIPV, the most frequently reported serious clinical diagnosis was thrombocytopenia (62.50%, n = 5; 61.91%, n = 13, respectively). According to the causality assessments, at least two thrombocytopenia cases were classified as related to vaccination with sIPV, and 10 thrombocytopenia cases were classified as related to vaccination with wIPV. One death was reported following sIPV administration, and three deaths were reported following wIPV administration; all of these deaths were identified as coincidental events in the causality assessment. Two allergic purpura cases were reported after vaccination with sIPV. One epilepsy case was reported after vaccination with wIPV. The other cases of wIPV-associated AEFIs were identified as coincidental events (Table 3).

Table 3. Serious adverse events following sIPV and wIPV vaccination, Jiangsu, China, 2016–2019

Clinical diagnosis	sIPV				wIPV
	No. (%^†)	Reporting rates*	Causality assessment		No. (%^†)	Reporting rates*	Causality assessment
			Coincidental	Consistent			Coincidental	Consistent
Thrombocytopenia	5(62.50)	0.38	3	2	13(61.91)	0.59	3	10
Allergic purpura	2(25.00)	0.15	0	2	0(0.00)	0.00
Death	1(12.50)	0.08	1	0	3(14.29)	0.14	3	0
Encephalitis and meningitis	0(0.00)	0.00			2(9.52)	0.09	2	0
Epilepsy	0(0.00)	0.00			1(4.76)	0.05	0	1
Encephalopathy	0(0.00)	0.00			1(4.76)	0.05	1	0
Sepsis	0(0.00)	0.00			1(4.76)	0.05	1	0
Total	8(100.00)	0.61	4	4	21(100.00)	0.95	10	11

*Reporting rates: per 100,000 doses administered.

^†%: % of all serious AEFIs.

4. Discussion

This study presents an evaluation of AEFIs reported to a passive surveillance system in Jiangsu, China, during the first 4 y after including the IPV vaccine in the EPI. The AEFI reporting rates for the newly licensed sIPV were compared to those of conventional wIPV.

The overall AEFI reporting rate for sIPV and wIPV were 53.02 and 41.25 per 100,000 doses, respectively. Compared with the monitoring data of two pilot provinces, the overall AEFI reporting rate for sIPV in this study is much lower than that reported by Fu S (168.91 per 100,000 doses in Jilin Province in 2016) and Shi X (2,464.33 and 53.35 per 100,000 doses by active and passive surveillance, respectively, in Ningxia Province during September 2015 to February 2016)^22,23 One possible explanation for this large difference is that active AEFI monitoring for sIPV was carried out in some areas in Jilin Province and Ningxia Province during the pilot period. The sensitivity of active monitoring is much higher than that of passive monitoring. Another possible explanation is that high social concerns had been paid on the new vaccine during the pilot period. However, the overall AEFI reporting rate for sIPV in this study is higher than that reported by Wang S (26.94 per 100,000 doses),²⁴ whose study based on the passive surveillance conducted in another pilot Province. This may be related to differences in the surveillance sensitivity, the sample size, and the observation period. wIPV has been widely used in some developed countries, but usually in the form of an IPV combination vaccine (e.g., DTaP-IPV, DTaP–Hib–IPV, DTaP-IPV-HepB, DTaP-IPV-HepB-Hib, and Tdap–IPV).^8,9,27,28 Hence, there are only a few studies of AEFI surveillance following the administration of standalone wIPV. The overall AEFI reporting rates for standalone IPV were 0.14, 5.8, 1.6, and 1.1 per 100,000 doses in the USA, Spain, Oman, and South Korea, respectively.^8,29-31 The overall AEFI reporting rate for wIPV in this study is much higher than the international rates. The significant variation in AEFI reporting rates after wIPV vaccination in different regions and at different times may be explained by variable reporting requirements, case definitions, and monitoring sensitivity. The AEFI reporting rate for wIPV identified in the current study is also higher than that reported by LI K (l5.63 per 100,000 doses).³² This may be related to several factors: 1) LI K estimated the rate using the number of wIPV doses distributed nationwide throughout 2011–2013 as the denominator, and the number of distributed doses is greater than the number of administered doses, thus reducing the rate (underestimation); 2) the surveillance sensitivity in Jiangsu Province is higher than that of other regions of the country;³² 3) insufficient attention was given to adverse events following wIPV administration before it was included in the EPI.

Our comparison revealed that the overall AEFI reporting rate for sIPV was slightly higher than that for wIPV. Further analysis suggests that this slight difference is mainly manifested in the reporting rate of common adverse reactions. sIPV had a slightly higher reporting rate than wIPV of common adverse reactions. The differences between sIPV and wIPV in the reporting rates of rare adverse reactions and coincidental events were not significant.

Similar to other reports, common adverse reactions were the most common AEFIs for IPV, and the vast majority of AEFIs after IPV vaccination in this study were relatively minor, mild, and self-limiting.^22-24,32 Fever was the most common event reported after sIPV and wIPV vaccination. This finding is consistent with surveillance reports^22-24,32 and clinical trials.^18,19 Hu Y found that the occurrence of fever was more frequent in the sIPV group than in the wIPV group in phase 3 clinical trials.¹⁹ We also found that the reporting rate of fever after vaccination with sIPV was higher than that of wIPV in post-marking surveillance in a large population. The specific reason for this phenomenon is unknown, but it may be related to different production processes. Persistent crying was the second most frequently reported symptom from sIPV and wIPV vaccination. The incidence of persistent crying after wIPV vaccination was lower than after sIPV vaccination; this may be due to the different syringes and needles that were used in the vaccination. Pre-filled syringes produced by Becton, Dickinson, and Company (BD) were used to inject wIPV, while common syringes and needles were used to inject sIPV. Injection site erythema and/or swelling was the most common local symptom, which was similar to the results of several previous reports.^19,20

The adverse events following IPV vaccination were mainly non-serious. The proportion of serious AEFIs was very small. There was no statistically significant difference in the reporting rate of serious AEFIs between sIPV (0.61 per 100,000 doses) and wIPV (0.95 per 100,000 doses). Thrombocytopenia was the most frequently reported serious AEFI for both sIPV and wIPV, with a reporting rate of 0.38 and 0.59 per 100,000 doses, respectively. After a causality assessment by an expert committee, two thrombocytopenia cases were found to be related to sIPV vaccination, and 10 thrombocytopenia cases were found to be related to wIPV vaccination. Thrombocytopenia has been reported as a serious adverse event following IPV administration in surveillance reports³² and even in a clinical trial.²⁰ Previous studies have found that it is not uncommon to record a close temporal relationship between vaccination and sudden unexpected death in infancy as a coincidental event.^33,34 In the current study, three deaths were reported in the observation period, and all of them were identified as coincidental events through comprehensive causality assessment. In addition to thrombocytopenia and death, the serious AEFIs include allergic purpura and epilepsy. Generally, the incidence of serious AEFIs was very low after vaccination with either sIPV or wIPV. sIPV induced an acceptable immune reaction and had a safety profile consistent with that of wIPV.

This study has three major limitations. First, as a passive surveillance system, CNAEFIS has some inherent drawbacks, including potentially biased reporting (probable under-reporting or over-reporting), inconsistency in the quality and completeness of reports, and a lack of control groups.³⁵ Second, the description of mild signs and symptoms was based on the recollections of parents or adults who observed children during the episode and some of these descriptions were not verified by health professionals. Moreover, we did not apply the standard definitions of AEFI diagnosis created by the Brighton Collaboration.³⁶

In conclusion, our findings demonstrated that the majority of AEFIs following vaccination of sIPV and wIPV were common adverse reactions. The incidence of serious AEFIs following the administration of IPV was very low, and there was no difference between sIPV and wIPV. Although the reporting rate of common adverse reactions following sIPV administration was slightly higher than that of wIPV in this study, the overall reporting rate of AEFIs following sIPV is still within a reasonable range. Therefore, our findings indicate that sIPV has a favorable safety profile and can be widely used. It may contribute to achieving polio eradication in developing countries and maintaining a polio-free world.

Disclosure of potential conflicts of interest

No potential conflicts of interest were disclosed.

Author contributions

Conceived and designed the study: Jun Gao, Guodong Kang, Fenyang Tang. Obtained and organized the data: Guodong Kang, Ran Hu, Jun Gao. Analyzed the data: Jun Gao, Jing Yu. Contributed materials/analysis tools: Zhiguo Wang, Jing Yu. Wrote the manuscript: Guodong Kang, Jun Gao.

Acknowledgments

We are grateful to all health workers in all vaccine clinics for collecting AEFI reports and submitting vaccination dose information in Jiangsu province. We thank our colleagues of CDCs at the county and prefectural levels for examining and verifying AEFIs.

Special acknowledgments are given to Professor Liu Dawei, Chinese CDC, Beijing, China, for his guidance.

Additional information

Funding

This work was supported by the Chinese Preventive Medicine Association “vaccine and immunization young talent lift project” under Grant [No. CPMAQT_YM0228].