Serological investigation on the prevalence of poliovirus in Guangdong province: A cross-sectional study

ABSTRACT

In 2019, we conducted a cross-sectional study for polio virus seroprevalence in Guangdong province, China. We assessed the positivity rates of poliomyelitis NA and GMT in serum across various demographic groups, and the current findings were compared with pre-switch data from 2014. Using multistage random sampling method, four counties/districts were randomly selected per city, and within each, one general hospital and two township hospitals were chosen. Healthy individuals coming for medical checkups or vaccination were invited. A total of 1318 individual samples were collected and tested. In non-newborn population, age-dependent positivity rates ranged from 77.8% to 100% for PV1 NA and 70.3% to 98.9% for PV3 NA (p < .01). The lowest GMT values for both types (17.03 and 8.46) occurred in the 20 to <30 years age group, while peak GMTs for PV1 and PV3 were observed in 1 to <2 (340.14) and 0 to <1-year (168.90) age groups, respectively. GMTs for PV1 (P = .002) and PV3 (P = .007) in Eastern Guangdong were lower than those in the other three regions. Male participants showed higher GMTs than females (P = .016 and .033, respectively). In newborn population, both males and females showed higher PV1 NA positivity rates and GMTs compared to PV3 (p < .05). Post-switch PV3 NA positivity rates were higher than pre-switch rates (p = .016). GMTs of both PV1 and PV3 were significantly higher post-switch (p < .001). The positivity rates of NAs and GMTs remain high level, which play an important role in resisting poliomyelitis infection. Effect of the converted immunization program was more pronounced than that before.

KEYWORDS:

• Polio
• cross-sectional study
• serosurvey
• Guangdong
• China

Introduction

Polio, an infectious disease caused by three serotypes of poliovirus (PV1, 2, and 3), is particularly detrimental to children, potentially leading to lifelong disability.¹ Significant progress has been made in reducing global polio incidence since the launch of the Global Polio Eradication Initiative (GPEI) in 1988, with the exception of Pakistan and Afghanistan. Before the widespread use of vaccines, wild type 1 poliovirus was the primary cause of most cases of paralytic polio worldwide. Wild types 2 and 3 are considered eradicated as of now.² As well known, vaccination remains the primary strategy, utilizing strategies such as the incorporation of oral poliomyelitis vaccine (OPV) into the Expanded Program Immunization (EPI), sequential administration of inactivated poliomyelitis vaccine (IPV) to mitigate vaccine-associated paralytic poliomyelitis (VAPP) and vaccine-derived poliovirus (VDPV).^3–5

The People’s Republic of China was established in 1949, with polio cases being reported since the 1950s. The number of polio cases was as high as 10,000–30,000 every year during the 1960s based on incomplete statistical records.⁶ In 1994, the last indigenous case of wild poliovirus (WPV) infection was reported, and in October 2000, along with all other countries from the Western Pacific Region, China was certified to be free of polio.⁷ Located in Southern China, Guangdong is a populous and economically vibrant province known for its significant commercial and trade activities. Despite no reported WPV cases in Guangdong for over 20 years, its significant commercial activity necessitates the implementation of robust vaccination strategies to safeguard against potential imported WPV or local VDPV epidemics.

In 2013, the World Health Assembly endorsed the phased withdrawal of OPV and introduction of IPV into childhood routine immunization schedules. Type 2 OPV was withdrawn through a globally synchronized “switch” from trivalent OPV (all three types) to bivalent OPV (types 1 and 3).⁸ As a member of the World Health Organization, China has also responded to the call of the WHO and joined the rest of the world to implement the “switch” in 2016. Then, Guangdong has adapted its immunization regimen, initially adopting a “1 dose of IPV + 3 doses of bOPV” approach and subsequently shifting to “2 doses of IPV + 2 doses of bOPV” in 2019, which includes 3 doses for primary immunization and 1 dose for booster immunization. The recommended vaccination schedule for primary immunization is at 2, 3, and 4 months of age while for booster immunization is at 4 years old. Of course, these are National Immunization Program Vaccines (NIPV), and they are free of charge. Additionally, there is another scenario where all four doses are IPV (with the last two doses being self-funded), while the booster dose is administered at 18 months of age. Notable, IPV can be replaced with another product, called the pentavalent vaccine, produced by Sanofi Pasteur, which is officially known as DTaP-IPV/Hib pentavalent vaccine. It is a combination vaccine consisting of adsorbed acellular pertussis vaccine, inactivated poliovirus vaccine, and Haemophilus influenza type b conjugate vaccine. In mainland China, it is part of the Non-National Immunization Program Vaccines (NNIPV, fee-based). As it contains the IPV component, it can be used as a substitute for the NIPV-OPV or IPV. By receiving the pentavalent vaccine as required, the corresponding polio vaccine immunization program can be considered as complete.

Our study therefore assessed the positivity rates of poliomyelitis neutralizing antibodies (NA) and geometric mean titer (GMT) in serum across various demographic groups in the province. Regular assessments of these NAs and GMTs can reveal insights into risk groups, evaluate the relative effectiveness of different vaccination programs, establish NAs positivity rates and GMTs in various age cohorts, and monitor antibody transmission in neonates post-switch. Moreover, to elucidate the long-term effects of the vaccination switch, we also compared the current findings with pre-switch data from 2014.

Method

Specimen composition

In 2019, a serum survey was conducted across all 21 prefecture-level cities in Guangdong, China. Specimens came from 21 cities in the province, with at least 60 specimens collected from each city. The specimens need to cover 15 age ranges, including newborns, 0 to <1, 1 to <2, 2 to <3, 3 to <4, 4 to <5, 5 to <6, 6 to <7, 7 to <8, 8 to <9, 9 to <10, 10 to <20, 20 to <30, 30 to <40, and ≥40 (years old). We required each city to strive for an evenly distributed number of specimens in each age group during the collection process.

Specimen collection

Using multistage random sampling. Four counties/districts were randomly selected per city, and within each, one general hospital and two township hospitals or community healthcare centers were chosen. Healthy individuals coming for medical checkups at the hospital or receiving vaccinations at the community healthcare centers were invited and asked for their consent to participate in the survey, especially excluding those with immunodeficiency, travel history to high-risk countries including Pakistan, Afghanistan, and Nigeria within the past year, under immunosuppressant treatment, or receiving immunoglobulin injection within the past 3 months. Data collected from eligible participants included personal details (gender, age, region) and, for younger cohorts, vaccination certificate numbers were also collected to provide access to their immunization histories if necessary.

Laboratory testing

Venipuncture was used to collect 2 ml blood samples from each individual (cord blood collection for newborns), which were immediately placed in ice boxes for transport to local laboratories. Post-centrifugation, the serum was extracted and stored at −20°C before being sent to Guangdong Provincial Center for Disease Control and Prevention’s polio lab for analysis. According to WHO guidelines, we used a microneutralization assay (MNA) to determine P1 and P3 neutralizing antibodies. Each serum sample was inactivated at 56°C for 30 minutes, diluted from 1:4 to 1:1,024, then incubated with 100 poliovirus antigen median tissue culture infective dose (TCID50) at 36°C for 3 hours. After 7-day culture period, the serum dilution degree that could protect 50% of the culture was noted. According to relevant study,⁹ IPV and OPV work in both parallel and different ways. Each vaccine elicits serum antibodies that prevent viremia, and neutralization at titers of 1/8 (or even 1/4) is protective. Thus, we thought that titer exceeding 1:4 was classified as positive in this study. Moreover, each test incorporated both cell control and standard serum tests to validate result reproducibility.

Statistical analysis

The data were collected by Excel 2013 and imported into spss 19 for analysis. Descriptive analysis was used to analyze the different distributions of specimen positive rate, chi-square test was used for comparison between groups, and analysis of variance after log transformation was used for comparison of serum antibody levels between groups, with level of significance determined at a .05 p value.

Ethical approval and informed consent

From May 6th to 8th, 2019, prior to initiation of the survey, the project team was required to submit a “Ethical Review and Approval Form for Human Biomedical Research (W96-027B)” to the “Medical Research Ethics Committee” of GDCDC, after a scientific evaluation by the committee, the research was approved to be conducted. Recruitment of subjects started after obtaining the committee approval and all information from participants was collected after their permission. Written informed consent was sought from each participant, and investigators should explain to them that their information would be only used to this research.

Result

Study population

Samples were collected and tested from a total of 1318 individuals. These were divided into age groups (in years), starting from newborns (collecting cord blood), through each subsequent year up to 10 years old, and then by decade for those aged 20 years and older. Finally, there were 89 samples in the newborns group and 1229 in the non-newborns group. The samples represented 21 prefecture-level cities in Guangdong province (Table 1) and had a nearly balanced gender distribution, with 651 males and 657 females, yielding a gender ratio of 1:1.01.

Table 1. Distribution of samples from 21 prefecture-level cities by age.

Cities	Age Groups (years)
	Newborns	0 to <1	1 to <2	2 to <3	3 to <4	4 to <5	5 to <6	6 to <7	7 to <8	8 to <9	9 to <10	10 to <20	20 to <30	30 to <40	≥40	Total
Chaozhou	4	4	4	4	4	4	5	3	4	3	5	4	4	4	4	60
Dongguan	3	4	4	3	4	4	4	4	4	4	4	4	4	4	4	58
Foshan	4	4	4	4	4	4	4	4	4	4	4	4	4	4	4	60
Guangzhou	9	0	0	7	4	7	5	4	4	4	5	2	4	4	4	63
Heyuan	4	4	4	4	4	6	2	5	5	3	5	3	4	4	4	61
Huizhou	4	4	6	6	6	6	6	7	7	6	5	5	6	4	4	82
Jiangmen	4	6	4	4	4	5	5	4	6	5	4	10	6	4	4	75
Jieyang	4	4	4	4	4	4	4	4	3	5	4	4	6	3	3	60
Maoming	4	4	4	4	4	4	4	4	4	4	4	4	4	4	4	60
Meizhou	4	5	3	4	4	4	7	4	7	4	4	4	5	3	4	66
Qingyuan	4	4	4	5	4	4	4	4	6	4	4	5	4	4	4	64
Shantou	4	5	3	5	6	6	6	9	5	6	4	5	5	5	5	79
Shanwei	4	8	4	4	4	3	2	2	2	3	2	2	3	4	4	51
Shaoguan	4	4	4	4	4	4	4	4	4	4	4	4	4	4	4	60
Shenzhen	4	3	4	4	4	4	4	4	4	4	4	4	4	4	4	59
Yangjiang	4	4	3	5	4	4	4	4	4	5	3	4	3	5	4	60
Yunfu	4	4	4	4	4	3	5	4	4	4	5	3	4	4	4	60
Zhanjiang	4	4	4	4	5	3	5	3	2	6	4	4	4	3	4	59
Zhaoqing	4	2	5	2	5	4	1	8	4	3	4	6	4	4	4	60
Zhongshan	5	4	4	5	3	4	4	4	3	5	3	5	4	4	4	61
Zhuhai	4	4	3	5	3	5	4	5	3	4	4	5	5	2	4	60
Total	89	85	79	91	88	92	89	94	89	90	85	91	91	81	84	1318

Distribution of PV antibody level by age (excluding newborns)

Out of 1229 samples, 95.4% (1173) tested positive for PV1 NA and 90.8% (1116) for PV3 NA. The concurrent positivity rates for both tests were 89.0% (1094), while the rates of both tests being negative were 2.8% (35). PV1 NA positive rate was higher than that of PV3 NA (p < .01), with GMTs of 89.88 and 35.51, respectively (p < .01). Age-dependent positivity rates ranged from 77.8% to 100% for PV1 NA and 70.3% to 98.9% for PV3 NA, signifying significant age-related variance (p < .01). PV1 NA positivity rate was lowest in the 30 to <40 years age group (77.8%), peaking in 5 to <6, 7 to <8, and 9 to <10 years age groups (100%). Conversely, PV3 NA showed the least positivity rate in the 20 to <30 years age group (70.3%), highest in the 2 to <3 years age group (98.9%). The lowest GMT values for both types occurred in the 30 to <40 years age group (15.35) and 20 to <30 years age group (8.46), while peak GMTs for PV1 and PV3 were observed in 1 to <2 years age group (340.14) and 0 to <1 years age groups (168.90), respectively (Table 2).

Table 2. Distribution of PV1 and PV3 neutralizing antibodies (NA) positivity rates and geometric mean titers (GMT).

Projects	Sample number	PV1 NA positive number	PV1 NA positive rate	P value	PV3 NA positive number	PV3 NA positive rate	P value	P value	PV1 GMT	P value	PV3 GMT	P value	P value
Age groups (years)
0 to < 1	85	83	97.6%		78	91.8%			304.44		168.90
1 to < 2	79	78	98.7%		78	98.7%			340.14		162.02
2 to < 3	91	89	97.8%		90	98.9%			284.05		157.59
3 to < 4	88	86	97.7%		84	95.5%			134.36		60.13
4 to < 5	92	91	98.9%		90	97.8%			165.42		58.08
5 to < 6	89	89	100.0%		86	96.6%			165.42		49.18
6 to < 7	94	92	97.9%		90	95.7%			100.43		31.78
7 to < 8	89	89	100.0%		84	94.4%			97.68		33.36
8 to < 9	90	88	97.8%		85	94.4%			82.71		32.45
9 to < 10	85	85	100.0%		84	98.8%			79.89		20.82
10 to < 20	91	90	98.9%		82	90.1%			53.82		13.83
20 to < 30	91	78	85.7%		64	70.3%			17.03		8.46
30 to < 40	81	63	77.8%		59	72.8%			15.35		9.13
≥40	84	72	85.7%		61	72.6%			21.71		9.38
Area				=.876			=.947			=.002		=.007
Pearl river delta	537	514	95.7%		489	91.1%		=.002	97.68		41.07		＜.001
Eastern Guangdong	234	224	95.7%		210	89.7%		=.013	64.45		26.35		＜.001
Western Guangdong	223	213	95.5%		203	91.0%		=.059	81.57		30.48		＜.001
Northern Guangdong	235	222	94.5%		213	90.6%		=.114	111.43		39.67		＜.001
Gender				=.828			=.481			.016		.033
Male	610	583	95.6%		557	91.3%		=.003	100.43		39.40		＜.001
Female	619	590	95.3%		558	90.1%		＜.001	79.90		32.00		＜.001

Distribution of PV antibody level by region

Our study included participants from 21 cities across Guangdong, categorized into four regions: Pearl River Delta, Eastern Guangdong, Western Guangdong, and Northern Guangdong. PV1 and PV3 NAs positivity rates showed no significant regional differences (P = .876 and P = .947, respectively). Comparisons between PV1 and PV3 NAs positivity rates were also statistically insignificant in Western and Northern Guangdong (P = .059 and P = .114, respectively), while the Pearl River Delta and Eastern Guangdong showed higher rates for PV1 (P = .002 and P = .013, respectively). GMTs for PV1 (P = .002) and PV3 (P = .007) in Eastern Guangdong were lower than those in the other three regions. Northern Guangdong boasted the highest PV1 GMT (111.43), with Pearl River Delta leading in PV3 GMT (41.07). PV1 GMT consistently showed higher than PV3 GMT across all regions (p < .001) (Table 2).

Distribution of PV antibody level by gender

There is no significant gender difference in PV1 or PV3 NAs positivity rates (P = .828 and P = .481, respectively). However, male participants showed higher GMTs than females (P = .016 and P = .033, respectively). Regardless of gender, PV1 NAs positivity rates and GMTs were higher than those of PV3 (P = .003, P < .001 for positivity rates; P < .001 for GMTs). (Table 2)

Distribution of PV NAs and GMTs of newborns in different regions and genders

Of 89 newborns cord blood samples collected, PV1 and PV3 NAs positivity rates and GMTs showed no significant regional variation (P > .05). In the Pearl River Delta, PV1 NA positivity rate and GMT were significantly higher than those of PV3 (P = .007 for positivity rates, P = .002 for GMTs), while no such difference was observed in other regions (P > .05). The PV1 or PV3 NA positivity rates or GMTs did not vary by sex (p > .05). However, both male and female newborns showed higher PV1 NAs positivity rates and GMTs compared to PV3 (P = .004, P = .009 for positivity rates; P = .005, P = .006 for GMTs, respectively). (Table 3)

Table 3. Regional and gender-based distribution of poliovirus neutralizing antibodies (NA) and geometric mean titers (GMT) in newborns.

Projects	Sample number	PV1NA positive number	PV1NA positive rate	P value	PV3NA positive number	PV3NA positive rate	P value	P value	PV1 GMT	P value	PV3 GMT	P value	P value
Area				=.856			=.906			=.590		=.907
Pearl river delta	41	35	85.4%		24	58.5%		=.007	15.24		6.32		=.002
Eastern Guangdong	16	13	81.3%		8	50.0%		=.135	13.54		8.00		=.094
Western Guangdong	16	12	75.0%		9	56.3%		=.458	8.34		5.66		=.217
Northern Guangdong	16	13	81.3%		10	62.5%		=.217	16.00		7.36		=.122
Gender				=.687			=.312			=.631		=.351
Male	46	37	80.4%		24	52.2%		=.004	11.63		6.50		=.005
Female	43	36	83.7%		27	62.8%		=.009	15.78		6.82		=.006

Comparison of PV NAs positive and GMTs on children aged 0–3 years before and after the change of polio immunization program

To evaluate the vaccination effect of polio vaccine before (t-t-t) and after (i-o-o) conversion, a total of 353 children aged 0–3 were examined. This included 245 samples from pre-switch period (collected from the immunization level monitoring project in Guangdong Province conducted in 2014) and 108 from post-switch period (obtained from the serological survey conducted in this study). The PV1 NAs positivity rates were statistically consistent before (98.4%) and after (99.1%) the switch (P = .977). However, the post-switch PV3 NA positivity rate was higher than pre-switch rate (p = .016). Prior to the switch, the PV1 NA positivity rate was higher than that of the PV3 rate (p = .002), but a discrepancy was absent after the switch (P = 1.000). GMTs of both PV1 and PV3 were significantly higher in the post-switch period (p < .001). Regardless of the immunization program, PV1 GMT was higher than the PV3 GMT (P < .001). (Table 4)

Table 4. Comparative analysis of poliovirus neutralizing antibodies (NA) positivity rate and geometric mean titers (GMT) in children aged 0–3 years pre- and post-immunization program shift.

Year	Sample number	PV1NA positive number	PV1NA positive rate	P value	PV3NA positive number	PV3NA positive rate	P value	P value	PV1 GMT	P value	PV3 GMT	P value	P value
Polio conversion				=.977			=.016			<.001		<.001
2014 (t-t-t)	245	241	98.4%		225	91.8%		=.002	192.67		50.21		<.001
2019 (i-o-o)	108	107	99.1%		107	99.1%		=1.000	415.87		199.47		<.001

Discussion

Our study examined positivity rates of poliomyelitis NAs and GMTs in serum and the effects of the modified polio vaccination program in Guangdong province. It showed that both PV1 NA and PV3 NA were present in a majority of tested participants, indicating a successful vaccination program. Moreover, we also showed that NAs positivity rates varied among different age groups significantly and the PV1 NA positive rate was consistently higher than PV3 NA across different demographic categories such as region, gender, and age groups. Additionally, the GMT was higher for PV1 than for PV3, and higher in males than in females. Newborns, however, did not show significant regional or gender differences in either NAs positivity rates or GMTs. The study also highlighted that the post-switch immunization program demonstrated greater efficacy, as evidenced by a significant increase in the positive rate of PV3 NA and GMT values for both PV1 and PV3 compared to pre-switch.

Our findings were consistent with results of a previous study conducted by Veronesi,¹⁰ showing the comparatively weaker immunogenicity of PV3 to PV1. This discrepancy may be attributable to the differential rates of antibody attenuation post-vaccination, with PV3 antibodies diminishing more rapidly than PV1.¹¹ Statistical significance was observed in the positivity rates and GMTs of PV1 and PV3 antibodies across different age cohorts. These measures peaked in the 1 to <2 and 2 to <3 years age groups before gradually declining, with a minor increase noted in the 4 to <5 years age group, possibly due to our regional polio vaccination schedule. The regimen currently employed in our province consists of an initial IPV dose at 2 and 3 months of age, followed by two doses of OPV at 4 months and 4 years of age, respectively. Our results reflect the stringent adherence to this immunization strategy across all regions in our province, as per intended objectives to effectively mitigate the risk of polio infection. However, the swift decline in population GMTs after the age of 10 years raises concerns, as it may potentially drop below the protective threshold for some participants. Yet, other studies¹² suggest that even in the face of low GMT levels many years post-vaccination, complete polio vaccination can still provide ongoing protection due to immune memory. This implies that, when naturally stimulated, a rapid immune response may be triggered to thwart infection.

While there were no regional disparities in the positive rates of PV1 and PV3 NAs, variations in GMTs were identified. Eastern Guangdong exhibited lower NAs and GMTs for both PV1 and PV3 compared to the other three regions, with northern Guangdong demonstrating the highest PV1 GMT and Pearl River Delta the highest PV3 GMT. The reasons for these findings in eastern Guangdong are unclear at present. Although an investigation in 2014 found the lowest PV1 and PV3 NAs and GMTs in western Guangdong,¹³ no unusual poliomyelitis or VAPV/VDPV incidence was reported in eastern or western Guangdong in recent disease surveillance. One hypothesis attributes these differences to weaker vaccination attitudes in these economically disadvantaged areas, which may stem from factors such as limited vaccination accessibility, lower educational levels among caretaking grandparents, low family income, and vaccine skepticism. These influences could disrupt adherence to regular vaccination procedures or discourage vaccination entirely. For example, in developed cities located in Pearl River Delta region such as Guangzhou and Shenzhen, the annual vaccination coverage rates for polio vaccine exceed 95%, and sometimes even reach 99%. In contrast, in some economically underdeveloped cities outside of the Pearl River Delta region, the vaccination rates often fail to reach 95%, and in some cases, it cannot even reach 90%. Furthermore, the use of fully self-paid IPV or full pentavalent vaccines as alternatives to the free polio vaccine is less frequent in economically disadvantaged areas. Whether this contributes to the lower NAs and GMTs in eastern Guangdong warrants further investigation, because a previous study¹⁴ indicated that children who received the fully self-paid IPV or full pentavalent vaccines exhibited higher NAs and GMTs than those receiving the first dose of OPV or full OPV. If validated, advocating for these superior vaccine varieties and immunization programs would be advisable. Irrespective of these findings, maintaining vigilance for WPV or VAPV/VDPV risk in eastern Guangdong remains crucial, as does the promotion of supplementary immunization activities and the potential adjustment of routine immunization and AFP surveillance as required.

Our study showed gender disparities in the positive rates of PV1 and PV3 NAs, aligning with findings from previous studies.^15,16 We further observed a significant difference in the GMTs, with males presenting significantly higher GMTs for both PV1 and PV3 than females. This difference may be attributable to the slower attenuation rate of poliomyelitis antibodies or a stronger immune response in males, resulting in enhanced antibody production when vaccinated under identical vaccine and immunization programs.

In the newborns group, we found no regional or gender disparities in the distribution of PV1 or PV3 NAs positive rates or GMTs. The Pearl River Delta region exhibited significantly higher PV1 NA positive rates and GMT in neonates compared to PV3, a distinction absent in the other three regions. Regardless of gender, the positive rate and GMT of PV1 surpassed those of PV3, implying superior immunogenicity of PV1 and enhanced maternal-fetal antibody transfer.¹⁷ This prompts a consideration of whether our current immunization strategy, which administers the first dose at 2 months, disproportionately emphasizes PV1 or combined antibody concentrations, possibly neglecting the actual titer levels of PV3. Notably, many children may have already depleted their maternally transferred PV3 antibodies before reaching 2 months of age.¹⁸ This synchronicity in the first-dose immunization schedule could be a consequence of various objective factors, such as combined antibody levels, vaccine manufacturing processes, and comprehensive vaccination programs, alongside cost-effectiveness metric. However, the prevailing immunization program remains efficacious in averting polio infection in children. The proposition to modify the immunization program based solely on our study’s outcomes is debatable, underscoring the necessity for additional research to substantiate this hypothesis. Additionally, it is worth mentioning that in the newborns group, cord blood samples were collected to avoid causing secondary harm. We did not compare the differences in antibody titers between cord blood and venous blood and did not find relevant literature discussing this aspect. However, Hartvigsson et al.¹⁹ suggested that cord blood is frequently used to monitor neonatal health and can be sampled for research purposes to predict and assess neonatal health problems. Another study²⁰ showed that collecting sufficient volumes of blood or other tissues from neonates and infants is excessively invasive compared with adults and is typically considered unethical. Therefore, previous studies took an alternative approach by collecting human cord blood as a substitute for peripheral blood from neonates and infants, which can be collected without causing pain or anemia in mothers and their babies and therefore can be considered as a cost-effective, humane resource for investigations of human neonatal and early infantile immunity. For example, Soysal et al.²¹ investigated the presence of anti-RBD antibodies in cord blood after the mother received a SARS-CoV-2 inactivated vaccine (CoronaVac) during pregnancy, a cord blood sample was collected immediately after birth and sent for detection of anti-RBD antibodies. Therefore, in this study, the results of cord blood testing instead of venous blood still have reference value.

We explored the impact of sequential polio vaccination programs on the basic immunization of children aged 0–3 years by comparing the 2014 (t-t-t) and 2019 (i-o-o) schedules. While the PV1 NAs positive rates remained consistent pre- and post-transition, we observed enhanced positive rate for PV3 NA and higher GMTs for both PV1 and PV3 post-transition. These results suggest the latter immunization program to be more effective in inducing PV3 antibody production, a finding corroborated by research suggesting improved antibody levels with an i-o-o sequence.²² This may be attributed to the removal of type 2 strains from bOPV, which could previously have been inhibiting type 3 strain activity or competitively interacting with them. Another plausible explanation could be the superior activity and immunogenicity of IPV’s type 3 strain compared to tOPV.²³ However, our study lacks a robust theoretical underpinning for this hypothesis, as we did not establish a separate control group for IPV but instead used it alternately with bOPV. Regardless, the latter immunization schedule deserves commendation for its efficacy and safety, echoing our findings in the neonatal group, the latest immunization schedule will likely help address the observed weaknesses in PV3 immunity.

In this study, we discussed the levels of polio antibodies in the population of Guangdong Province to evaluate immune effectiveness and the ability to resist polio infection. However, it is worth noting that as paralysis from wild poliovirus is eliminated, VAPP and VDPV become more prominent in countries that used only OPV, which is receiving increased attention as important risks from OPV, because as long as OPV is used, they will remain a risk.²⁴ For instance, an even more important fact was that ongoing cVDPV outbreaks had been reported in the Philippines, Sichuan province of China, the Democratic Republic of the Congo, and many other parts of the world.²⁵ Thus, the GPEI has to confront ongoing challenges to achieve the overarching goal, and therefore there is a critical need to maintain a highly sensitive surveillance system to detect WPV, especially VAPV and VDPV.

Limitations

Our study had some limitations. For example, the samples from hospital and community healthcare centers might not fully represent the general population. Nonetheless, we anticipate this potential bias is minimal due to the absence of wild poliovirus infections in our province for several years, indicating that the population’s immunization status is largely vaccine-derived. In addition, the collection of serum samples was obtained from various facilities of Guangdong province, and the pre-treatment of samples was performed at the local CDC without supervision from members of project team. If the preservation and treatment of samples were insufficient, it may have potentially affected the final results.

Conclusion

Our findings provide essential public health insights into polio vaccination strategies, which highlight the superior immunity conferred by the updated vaccination sequence (i-o-o) especially for PV3, suggesting a potential enhancement in population-wide immunity, providing a robust immune defense against poliomyelitis in Guangdong. Furthermore, observed disparities in GMTs and positive rates across regions, genders, and poliovirus types underline the need for tailored vaccination strategies and immediate action, pinpointing potential areas for enhancement.

Acknowledgments

We thank the staff of the 21 prefectural CDCs of Guangdong for conducting volunteer enrolment and specimen collection. We also thank the laboratory staff of the provincial CDC for testing the samples.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was supported by Guangdong Medical Science and Technology Research Foundation: Study on the immune effect of different Vaccine Immunization Strategies in Polio Vaccine Transformation stage [Grant No. A2017028].