Polio is a severe disease caused by a virus, primarily affecting children. In severe cases, it can lead to paralysis, respiratory failure, or even death. To prevent and control the spread of polio, scientists have long been devoted to the research and application of polio vaccines. In the early 1950s, American scientist Jonas Salk developed an inactivated polio vaccine, but it required a substantial amount of virus extraction and inactivation, making it expensive to produce and therefore not widely used. Later, Albert Sabin invented the oral polio vaccine, which uses live weakened viruses to stimulate the immune system, protecting the body from virus infection. With the introduction of the oral vaccine, global polio vaccination rates gradually increased, making it one of the world's most successful vaccines (Toodayan and Matteson, 2022).

Now, polio vaccines are widely used worldwide, including in China. According to the World Health Organization's data, in 2018, 29 countries worldwide administered a total of 550 million doses of polio vaccine, preventing a significant number of polio cases. In China, the polio vaccine is also part of the routine vaccination program and is available in hospitals and disease control centers at all levels. Additionally, the polio vaccine is also widely used for vaccination against specific populations, such as those involved in outdoor exploration and tourism. However, the polio vaccine also faces some challenges. The oral vaccine carries a certain risk of adverse effects and transmission due to the use of live weakened viruses, while the inactivated vaccine has a high production cost and is difficult to distribute globally.

Over the years, polio has posed significant challenges to public health worldwide. However, through vaccination, this review has made significant breakthroughs in controlling the spread of polio. Scientists are developing new types of polio vaccines, such as DNA vaccines and recombinant vaccines, which use new technologies and methods, providing better safety and efficacy prospects and potentially becoming the mainstream of future polio vaccines. With continuous advancements in scientific technology, the research and application of polio vaccines are continually evolving. This review aims to provide a better understanding of the research progress and current status of polio, enhancing awareness of polio and providing better protection for public health and safety.

1 Research on Polio Vaccines

1.1 Disease overview

Polio is an acute infectious disease caused by the poliovirus, primarily affecting children. The poliovirus belongs to the enterovirus genus and is an RNA virus. Humans beings are the only natural host for this virus, and infection can lead to different types of clinical presentations, including spinal cord poliomyelitis and brainstem poliomyelitis. Spinal cord poliomyelitis is the most common clinical presentation and with its main symptoms being limb paralysis, respiratory muscle weakness, and pain. Brainstem poliomyelitis, on the other hand, is rarer but often results in severe neurological complications and death. Polio is mainly transmitted through respiratory droplets or contact. Following infection, the virus replicates in the intestines and spreads to the nervous system through lymphatic channels and the bloodstream, causing neuronal damage and inflammatory reactions. The virus replicates and spreads rapidly, and once infected, it proliferates continuously, spreading rapidly throughout the body's organs and tissues, with the most severe damage occurring to the central nervous system (Figure 1).

Currently, poliomyelitis remains an important issue in the field of public health globally. Despite a significant reduction in the number of polio cases worldwide due to the promotion and widespread use of polio vaccines, this disease still poses certain risks and challenges. Particularly in impoverished regions and developing countries, the number of polio cases remains relatively high due to limitations in healthcare infrastructure and vaccine coverage. Poliomyelitis is a severe infectious disease that can have a profound impact on human health and lives. Although effective vaccines and control strategies are currently in place, cooperation and efforts from the Global Health Organization, governments, and scientists are still needed in the global process of controlling and eliminating polio. Only through global cooperation and joint efforts can we achieve the control and eradication of polio worldwide and make a greater contribution to the health and well-being of humanity.

1.2 Early research and development of polio vaccines

Early research on polio vaccines can be traced back to the 1930s. At that time, poliomyelitis was still a deadly infectious disease, particularly posing a greater threat to children. Against this backdrop, many scientists and medical experts began efforts to find more effective methods of prevention and treatment. One of the earliest researchers was the American vaccine scientist Jonas Salk. In the 1930s, Salk began researching methods for developing inactivated polio vaccines. His research focused on chemically inactivating the virus to make it a safe and effective vaccine. With Salk's efforts, they have developed an inactivated vaccine against polio, that could stimulate the human body's immune response, effectively preventing poliomyelitis.

Simultaneously, American researcher Albert Sabin was also dedicated to polio vaccine research. Sabin's focus was on developing an oral live attenuated vaccine. Through studying the biological characteristics and replication process of the virus, he successfully created an oral live attenuated polio vaccine. This vaccine was not only safe and easy to administer but also capable of eliciting long-lasting immune responses, effectively preventing poliomyelitis. Under the efforts of Salk and Sabin, both the inactivated and oral live attenuated polio vaccines underwent continuous improvement and refinement. In the 1950s and 1960s, these two vaccines were subjected to large-scale clinical trials and promotion worldwide. The trial results showed that both vaccines were highly safe and effective in preventing poliomyelitis transmission.

1.3 Promotion and widespread use of vaccines

Since the successful development of polio vaccines, there have been extensive efforts to promote and ensure the widespread use of these vaccines worldwide. Various measures have been taken by the World Health Organization (WHO) and other international organizations regarding vaccine production and supply. For example, they collaborate with pharmaceutical companies to promote vaccine production and supply to ensure an adequate vaccine supply. Furthermore, they cooperate with governments of different countries to strengthen the regulation and control of polio vaccines, preventing counterfeit and substandard vaccines from entering the market. Regarding vaccine coverage, governments and public health organizations have also implemented various measures. For instance, in some impoverished regions and developing countries, governments and organizations organize special vaccination campaigns to provide free vaccination services for children. Additionally, they use various media channels to educate the public on the importance and safety of polio vaccines, as well as how to administer vaccines correctly.

As a result of these efforts, the global vaccine coverage for polio has significantly improved. According to data from the World Health Organization, the number of polio cases has decreased by over 99% since 2000, primarily due to the promotion and widespread use of vaccines. However, despite significant progress in the promotion and widespread use of polio vaccines, challenges and difficulties still exist. For example, in some conflict-ridden, impoverished, and disaster-stricken areas, limitations in healthcare resources and infrastructure continue to pose difficulties in vaccine administration. Additionally, some members of the public, especially on social media, harbor doubts and misconceptions about vaccine safety, which can affect vaccine coverage. The promotion and widespread use of polio vaccines are a global collaborative process that requires the collective efforts of all parties. By strengthening vaccine production and supply, improving vaccine coverage, and enhancing public awareness and education, it is believed that China can better prevent and control the occurrence and transmission of poliomyelitis, making a greater contribution to the health and well-being of the global population (Orenstein and Ahmed, 2017).

2 Classification and Characteristics of Polio Vaccines

2.1 Live attenuated and inactivated vaccines

Live attenuated vaccines are vaccines containing live viruses that have been altered or weakened to reduce their virulence. These vaccines can stimulate long-term immune responses in the human body, providing prolonged protection. Generally, live attenuated vaccines need to be stored in a refrigerator to ensure their viability. The advantage of live attenuated vaccines is that they can generate relatively long-lasting immune responses, but they also come with higher risks and potential side effects. Therefore, live attenuated vaccines are contraindicated for certain populations, such as individuals with weakened immune systems (Figure 2).

Figure 2 Polio Live Attenuated Vaccine (Source: https://www.360zhyx.com/home-research-index-rid-72053.shtml)

Inactivated vaccines are a type of vaccine that contains viruses or pathogens that have been inactivated or killed during the preparation process. In the production of inactivated vaccines, the viruses are rendered nonfunctional through the use of chemicals or other methods to ensure that they cannot cause the disease. Inactivated vaccines are a safer choice for individuals with a weaker immune system. However, the duration of immunity provided by inactivated vaccines is relatively shorter, requiring periodic vaccinations to maintain their effectiveness (Figure 3).

Figure 3 Inactivated Polio Vaccine (Source: https://yi.9939.com/76809/sms.html)

It's important to note that different types of vaccines use different techniques and methods in their preparation, which can lead to variations in vaccine effectiveness and safety. For example, the preparation process of inactivated vaccines may alter the antigenicity of the virus, affecting their immunogenicity. Therefore, when choosing a vaccine type, it's necessary to consider individual characteristics and follow a doctor's advice. The classification and characteristics of polio vaccines are significant for the prevention and control of polio. Both live attenuated and inactivated vaccines have their advantages and disadvantages, and the choice of vaccine type should be based on individual circumstances. It's essential to follow a doctor's recommendations during the vaccination process and be aware of vaccine side effects and contraindications to ensure the safety and effectiveness of vaccination (Chen et al., 2021).

2.2 Antigens and preparation methods of vaccines

The process of preparing inactivated polio vaccines involves cultivating the poliovirus in cell cultures, and then using chemical substances or thermodynamic methods to inactivate the virus, ultimately producing the inactivated vaccine. During the preparation of inactivated vaccines, it is essential to ensure the integrity and stability of the virus while also ensuring that the inactivation process does not affect the virus's immunogenicity. The preparation method of live vaccines: The preparation method of live vaccines is relatively complex and requires multiple passages to gradually lose their virulence, ultimately leading to the preparation of live vaccines. During the preparation process, it is crucial to pay attention to the number of passages and cultivation conditions of the virus to ensure both its stability and immunogenicity (Figure 4).

Figure 4 The cells of the poliovirus causing spinal cord grey matter disease under a microscope (Source: http://cs.gzedu.com/jiaoshijixu/2011/hjyjk/chapter1/Resource/psk0801tz02.files/frame.htm)

During the vaccine preparation process, quality control measures are essential to ensure the safety and effectiveness of the vaccine. Vaccine quality control involves multiple steps, including virus purification, testing, and determination of virus content. Researchers are continuously exploring new preparation methods and technologies to enhance vaccine efficacy and safety. For example, in recent years, genetic engineering techniques have been gradually applied to the vaccine preparation process to improve vaccine effectiveness. Additionally, the vaccine preparation process must take into account the variability of the virus in different regions and populations to ensure vaccine efficacy.

The antigen of the poliovirus vaccine is the poliovirus itself, and strict quality control is required during the preparation process. The preparation methods for inactivated vaccines and live vaccines differ and should be chosen based on virus characteristics and preparation techniques. With the continuous advancement of technology, vaccine preparation methods are constantly updated and improved to enhance vaccine efficacy and safety.

2.3 Immune protection mechanism of vaccines

The immune protection mechanism of the poliovirus vaccine is to activate the human immune system to generate antibodies and cellular immune responses, thereby preventing the occurrence of poliomyelitis. The antigens in the vaccine stimulate the human immune system to produce antibodies and cellular immune responses. When the human body is exposed to the poliovirus, the immune system has already produced the corresponding antibodies and immune cells capable of recognizing and clearing the virus, thus preventing further infection and replication of the virus. At this point, the human body has established immune protection against the poliovirus, effectively preventing the occurrence of poliomyelitis.

The immune protection mechanism of vaccines is a complex process involving the interaction of multiple immune cells and molecules. Antibody-mediated immune protection: Antigens in the vaccine can activate the body's B cells to produce antibodies against the virus. These antibodies can bind to the virus, preventing further infection in the body. This process is known as antibody-mediated immune protection. Cell-mediated immune protection: Antigens in the vaccine can also activate the body's T cells, leading to cell-mediated immune protection. T cells can recognize and eliminate viruses infecting the body, thereby preventing further virus infection and replication. Immunological memory: After vaccination, the body's immune system establishes memory against the virus. This immunological memory allows the body to produce antibodies and immune cells more rapidly upon reexposure to the virus, leading to more effective prevention of virus infection and replication.

The immune protection mechanism of the poliovirus vaccine involves activating the body's immune system to produce antibodies and cell-mediated immune responses, thereby preventing poliomyelitis. The immune protection provided by vaccines is a complex process involving interactions between various immune cells and molecules. It's important to note that vaccine-induced immune protection is not absolute, so personal hygiene and disease prevention measures should still be observed after vaccination (Kew et al., 2005; Zhang et al., 2022) (Figure 5).

Figure 5 The Immunological Process of Vaccination In the Body (Source: https://blog.sciencenet.cn/blog-347754-1285946.html)

3 Innovation and Optimization of Vaccines

3.1 Innovation and improvement of vaccine technologies

With the continuous development of biotechnology and vaccine technology, vaccine preparation techniques have also been innovated and improved. In the research of polio vaccines, researchers have utilized genetic engineering techniques to insert a portion of the poliovirus's genetic sequence into a viral vector, creating a novel gene-recombinant vaccine. Compared to traditional vaccines, gene-recombinant vaccines offer better immunogenicity and safety, making them an important focus in vaccine research. By continuously innovating and improving vaccine preparation techniques and methods, researchers can enhance the immunogenicity and safety of vaccines, thereby better preventing and controlling the occurrence of infectious diseases like polio.

The preparation method of gene-recombinant vaccines involves combining a portion of the virus's genetic sequence with a different viral vector, forming a recombinant virus. Compared to traditional vaccines, gene-recombinant vaccines have higher immunogenicity, which enhances their immunogenic effectiveness. Additionally, gene-recombinant vaccines exhibit greater safety, reducing the occurrence of adverse reactions post-vaccination. Researchers are also exploring other vaccine preparation technologies and methods. For example, some researchers are developing vaccines based on DNA and RNA technologies. These vaccines activate the human immune system by directly introducing DNA or RNA molecules, resulting in the production of antibodies and cellular immune responses against the virus. These novel vaccines have advantages such as simplicity in preparation, high safety, and versatility, making them a significant direction for future vaccine development. Researchers are also working on improving vaccine stability and storage conditions to address transportation and storage issues more effectively. For instance, some researchers are developing new vaccine materials and storage containers to enhance vaccine stability and shelf life (Mohamed et al., 2022).

3.2 Optimization of vaccine immunization strategies and procedures

In order to enhance the immunogenicity of polio vaccines and increase their widespread coverage, researchers and public health agencies are continuously optimizing vaccine immunization strategies and procedures. Firstly, researchers are exploring various vaccination methods to improve the vaccine's immunogenicity. For instance, some researchers are studying vaccine dosages and intervals to determine the optimal vaccination approach. Studies have shown that increasing vaccine dosages and extending the time between vaccine doses can enhance vaccine effectiveness. Additionally, researchers are also investigating new vaccination methods, such as oral vaccines and nasal spray vaccines, to improve vaccine delivery. Furthermore, public health agencies are consistently promoting vaccine immunization to increase vaccination rates and prevent vaccine-related diseases. For example, in some regions, governments have implemented policies for free universal vaccination to boost vaccination rates. Moreover, certain areas have organized vaccination awareness campaigns to increase public knowledge and acceptance of vaccination.

In terms of vaccine administration procedures, researchers and public health agencies are continually optimizing processes to improve the efficiency and coverage of vaccine immunization. For instance, some regions have implemented electronic appointment and vaccine reminder systems to better manage the vaccination process and records. Additionally, some researchers are exploring new vaccination sites and modes, such as conducting vaccination activities in community health centers and schools, to better meet the public's vaccination needs. The optimization of vaccine immunization strategies and procedures is one of the crucial approaches to enhancing the immunogenicity of polio vaccines and ensuring widespread vaccination. By continuously exploring new vaccination methods and improving vaccination procedures, researchers and public health agencies can better address the challenges of vaccine administration, thereby providing enhanced protection for public health and safety (Zeng and Chen, 2019; Liu et al., 2021).

3.3 Global Control and Eradication of Vaccines

International organizations and public health institutions have intensified their collaboration in vaccine administration and promotion. For instance, organizations like WHO and UNICEF actively promote global vaccine immunization initiatives, providing free vaccination services for children worldwide. Additionally, these organizations conduct vaccine promotion and awareness campaigns to enhance public understanding and acceptance of vaccination. Cooperation has been reinforced in vaccine storage and transportation by international organizations and public health institutions. For example, these entities offer guidance and support for vaccine storage and transportation to ensure vaccine quality and safety. Simultaneously, they engage in the construction and enhancement of vaccine transportation and storage facilities to improve vaccine stability and shelf life (Platt et al., 2014). International organizations and public health institutions have also bolstered their collaboration in vaccine monitoring and assessment. For instance, they conduct monitoring and assessment of vaccine immunization effectiveness to gauge vaccination coverage and impact. Furthermore, these organizations carry out monitoring and assessment of adverse vaccine reactions to guarantee vaccine safety.

The global control and eradication of vaccines are integral components of public health initiatives. Vaccines play a crucial role in controlling and eliminating infectious diseases such as poliomyelitis. To achieve global control and elimination of vaccines, international organizations and public health institutions have implemented various measures. International organizations and public health institutions have strengthened cooperation in vaccine research and production. For instance, entities like the World Health Organization (WHO) and the United Nations Children's Fund (UNICEF) have increased their oversight and guidance of vaccine manufacturers to ensure vaccine quality and safety. Simultaneously, these organizations actively support vaccine research and innovation to enhance vaccine immunogenicity and safety. The global control and eradication of vaccines are vital components of the public health sector. By enhancing collaboration among international organizations and public health institutions, developing safer and more effective vaccines, promoting global vaccination initiatives, improving vaccine immunogenicity and safety, addressing vaccine storage and transportation, and monitoring and assessing vaccine immunization effectiveness and safety, China is poised to ultimately achieve global control and eradication of infectious diseases like poliomyelitis (Nathanson and Kew, 2010; Modlin et al., 2021).

4 Significance and Value of Vaccine Research and Application

The significance and value of vaccine research and application are profound. Firstly, vaccines are crucial tools for preventing infectious diseases, effectively safeguarding human populations from illness. Secondly, research into and the application of vaccines not only improve public health but also promote societal and economic development. For instance, through vaccination, China can effectively control and eliminate diseases, reducing the consumption of public medical resources and the burden of societal costs. Furthermore, vaccine research and application stimulate the advancement of science and technology, driving progress in the fields of medicine and biology (Paul and Priya, 2004).

Regarding the polio vaccine, research and application also hold significant meaning and value. Vaccine research and application can create economic benefits and societal welfare. Vaccination can reduce the impact of the diseases on the workforce and productivity, thereby enhancing societal productivity and economic benefits. Furthermore, vaccine research and application also can promote scientific education and raise public health awareness, increasing public understanding and importance placed on disease prevention and control. The research and application of the polio vaccine can also provide valuable lessons and references for the prevention and control of other diseases, furthering the development and progress of global public health endeavors.

The significance and value of vaccine research and application are multifaceted, including disease prevention, promoting economic and social development, advancing scientific and technological progress, and enhancing public health awareness, and so on. Therefore, continued support and investment in vaccine research and application are essential, with a focus on ongoing improvements and innovations to better safeguard public health and safety. Vaccine research and application represent a long-term and critical endeavor. While China has made significant strides in vaccine research and application, there is an ongoing need for innovation and enhancement to address existing issues and challenges, ultimately providing improved protection for public health and safety in the future.

From the perspective of the development history of polio vaccines, the research and application of vaccines have undergone years of effort and exploration. Since the 1950s, people have been studying polio vaccines and gradually mastering the production techniques and application methods. With the continuous advancement of technology, researchers have continuously improved and optimized the vaccine manufacturing processes and immunization strategies, leading to significant improvements in vaccine safety, immunogenicity, and coverage rates. Currently, polio vaccines have become safe and effective vaccines, widely used in polio prevention and control efforts worldwide. Through vaccination, China has successfully controlled and eliminated the majority of polio cases, providing essential protection for public health and safety (Zahra, 2022).

Although significant progress has been made in the current efforts to control polio through vaccination, China still faces numerous challenges and difficulties in its future development. The sustainability and reliability of vaccines need continuous improvement. While polio vaccines have demonstrated excellent immunogenicity, there are still limitations in certain special circumstances, such as immunocompromised individuals and vaccine adverse reactions. Therefore, China needs to continuously refine vaccine manufacturing processes and immunization strategies to enhance the immunogenicity and reliability of vaccines. Additionally, China also needs to establish a robust vaccine monitoring and evaluation system to promptly identify and address issues related to vaccine efficacy and immunological persistence.

The future trends and challenges in the development of polio vaccines will be one of the important issues in the field of medicine. China needs to strengthen international cooperation to jointly promote vaccine innovation and development in order to protect public health and safety. China should enhance collaboration between vaccine research institutions and manufacturing companies, improve supervision and guidance over vaccine production and distribution, and elevate the quality and safety of vaccines. Additionally, China also needs to intensify vaccine promotion and advocacy efforts, enhance public awareness and trust in vaccines, and increase vaccine coverage rates. With collective efforts in China, it is believed that future research and application of polio vaccines will achieve even more significant accomplishments and advancements, further safeguarding human health and well-being.

Acknowledgments

I would like to express my gratitude to Ms. Mengting Luo for her invaluable support throughout the entire process of creating this paper, enabling me to gain a deeper understanding of the development and current status of polio vaccines. I would also like to extend my thanks to the research grant provided by the Cuixi Biotechnology Research Institute.

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