Does Vaccination Increase the Risk of Autism Spectrum Disorder?

Autism spectrum disorder (ASD) is a developmental disorder that can cause significant social, communicative, and behavioral difficulties. With autism rates rising dramatically in recent years, researchers and concerned parents have theorized the causes of autism, and the subject has received much attention. Is the high rate of autism now due to increased diagnosis and reporting, changing autism definitions, or a rise in the number of people with ASD? People started to blame vaccines as a cause of the increased number of people with ASD. Vaccines and their connection to autism have been the subject of continuous debate. Some parents are concerned that vaccines, particularly the measles-mumps-rubella (MMR) vaccine and preservatives used in other childhood vaccines, may play a role in developing autism in their children. This systemic review explores the link between vaccination and autism in children. We conducted a literature search using PubMed and Google Scholar. We included papers written in the English language from 1998 to 2022, conducting human research that examines the relationship between vaccination and the development of autism using appropriate quality assessment tools. Two reviewers independently reviewed the content of the included studies. In total, 21 studies were deemed eligible.


Introduction And Background
The incidence of autism spectrum disorder (ASD) has risen substantially. This rise has sparked widespread public concern regarding the causes and prevention of the condition. The prevalence of ASD among children aged six to 11 years was 3 per 10,000 in 1991-1992 which increased to 52 per 10,000 in 2001-2002 [1].
Understandably, parents of children with the condition are often angry, feeling guilt, searching for causes, and asking themselves, "Why has this happened?" Many parents blamed themselves, claiming that the problem may be due to dangerous behavior during pregnancy, advanced age at conception, or a genetic element. A narrative that blames an external aspect, on the contrary, appears to be more comfortable; vaccines were the ideal target for their rage and frustration [2].
Wakefield et al. [3] published a report in 1998 describing 12 cases of widespread developmental slowdown linked to gastrointestinal (GI) system symptoms and developmental delay, a fair amount of which occurred shortly after the administration of measles, mumps, and rubella (MMR) vaccine. The theory presented in this case series was that a new variety of ASD was developing and linked to the MMR vaccine; this raised concerns among parents regarding the MMR vaccine's safety and vaccination in general [4,5]. Moreover, a few studies have correlated the number of vaccines added to the children's immunization schedule with the prevalence of ASD diagnosis. Mercury toxicity and modification in immune system function have been the subject of numerous investigations [6]. Following the article by Wakefield et al. on the MMR vaccinationautism link, there has been an upsurge in the antivaccine attitude and vaccine hesitancy in the United States. A lack of readiness to embrace immunization, as shown with pertussis immunization in many countries in the 1970s and 1980s and MMR immunization in the United Kingdom and the United States, resulted in the re-emergence of vaccine-preventable diseases. In the aftermath, acceptability improved [5]. The ideas in the report continue to raise anxiety and challenge vaccine acceptance among parents [6,7].
This systemic review aims to determine any relationship between vaccination and ASD development. We will review multiple articles on vaccination/MMR/ASD and understand their correlation.

Methodology
We conducted our systematic review utilizing the Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA) guidelines [8].

Database
We started our research on November 15, 2021. We used PubMed and Google Scholar as databases for our data collection.

Search strategy
We included studies on immunization/vaccination and autism/ASD. Our search mechanism included keywords and Medical Subject Headings (MeSH). Table 1

Inclusion criteria
We choose peer-reviewed papers and studies from the last five years written in the English language. We only selected systematic reviews, traditional reviews, meta-analyses, and randomized trials conducted among human subjects. All data collected were within ethical and legal standards.

Exclusion criteria
We excluded gray data and papers that focused on animals. We also excluded articles published before 1998.

Quality assessment tool
We used the following quality assessment tool to evaluate the papers utilized in this study: a Measurement Tool to Assess Systematic Reviews (AMSTAR) questionnaire for systematic reviews and meta-analysis, Cochrane risk bias assessment tools for randomized control trials, the scale for the Assessment of Narrative Review Articles (SANRA) for traditional reviews, and Newcastle-Ottawa Scale for observational studies. We discarded poor-quality studies.

Data collection
We collected the data from the selected articles (with high quality) individually.

Results
A total of 13,890 records were identified from both Google Scholar and PubMed database ( Table 1). We screened 266 records, of which 184 were excluded. After removing duplicates, 600 articles were considered ineligible by automation tools, and 134 were removed for other reasons. We were left with 82 reports for retrieval, 60 were not retrieved, leaving 22 articles. Subsequently, based on the exclusion criteria, we included 21 studies in our study. Figure 1 shows the PRISMA flow diagram.   Farrington et al. [14] Self-matched case series: compared rates of regression, parental concern, or diagnosis of ASD in specified periods after vaccination to all other periods for that individual (extended analysis of Taylor et al.)

2001
No increased incidence of diagnosis of ASI, regression, or parental concern 24, 36, or 60 months after vaccination; no increased likelihood of ASD, regression, or parental anxiety after immunization compared with before immunization Dales et al. [15] Time

Outcomes
In total, 19 articles were on the link between immunization and the incidence of autism. One article discussed the prevalence of ASD, the other about the effect of multiple immunizations during brain development (this study had data supporting the link between the current vaccine schedule and the development of ASD). The results of the 19 articles do not support a causal relationship between childhood immunization and the development of autism.

Discussion
ASD is a collection of phenotypic and developmental disorders resulting in significant social, communicative, and behavioral challenges. It is characterized by speech, language, and social functioning deficiencies and atypical behavioral symptoms, such as habitual, repetitive movements and extreme distress from environmental changes. Comorbidities include mental retardation, epilepsy, chronic gastrointestinal (GI) problems, and hyperactivity in certain persons. Parents of children with ASD frequently notice developmental difficulties during their child's first year of life. The disorder has a vital genetic component [1,5]. Many conditions that were formerly diagnosed as separate entities are now included in ASD diagnosis. These include Asperger syndrome, pervasive developmental disorder (PDD) not otherwise specified, and autistic disorder. ASD with regression is a subtype where patients with ASD have lost previously gained developmental skills, most commonly for language. Unfortunately, there is no cure or single diagnostic approach for the disorder, although some data suggest that early, intensive behavioral therapy may improve functioning [5].
In the late 1990s, Andrew Wakefield, a physician at London's Royal Free Hospital, published a paper in The Lancet claiming to have discovered the measles virus as the cause of autism. Initially, Wakefield stated that the measles virus caused colonic lesions in Crohn's disease. Although this idea was quickly debunked and dismissed, Wakefield was impressed by cases brought to his attention in which typically developing children developed autistic symptoms after receiving the MMR triple vaccine. Despite his prior miscalculation with Crohn's disease, he believed the measles virus had caused inflammatory lesions in the colon. All eight autistic patients on whom he had performed lower GI studies developed the hypothesized lesions, concluding that the measles vaccine virus led to the development of autism [2]. Wakefield's findings could have raised warning lights at this time, even if the measles virus turned out to be a coincidental cause of autism. Wakefield's claim was rapidly followed by reports of the detection of the measles virus in intestinal biopsies, blood, and cerebral spinal fluid samples taken from autistic children. After securing a straightforward and catchy scientific explanation, politicians and leaders of major groups of families of autistic children stood up with Wakefield.
In 2005, an investigative reporter brought to the attention of The Lancet's editors that Wakefield's study had been questioned by severe research misconduct, conflict of interests, and probably mendacity. After an inquiry into the subject, The Lancet withdrew the article, and the British Medical Association took strict actions against Wakefield. Since the Wakefield paper, any attempt to relate autism and the MMR vaccine has been disproved by many studies investigating the epidemiology of autism and the biological effects of MMR and the mumps virus. Mumps viruses were not regularly discovered in autistic children's natural materials at a higher incidence than in non-autistic youngsters. Furthermore, there was no evidence that a decrease in the rate of MMR exposure was associated with reductions in the incidence of autism. Regardless of scientific data, it was assumed that if the mumps virus was not to blame for autism, another MMR ingredient must be [2]. The substantial rise in the incidence of ASD has sparked widespread public concern regarding the disorder's causes and how to prevent them [1]. Despite extensive research about the etiology and pathophysiology of autism, few conclusions have been reached regarding a fundamental causal mechanism. There is no cure currently.
Several hypotheses were posited on the relationship between vaccination and autism development. The first theory relates to immune system dysfunction, organic acid synthesis, the effects of gliamorphin on cerebral function, and mercury toxicity. Parents are concerned about the safety of their children receiving numerous immunizations simultaneously. According to this theory, frequent stimulation of the systemic immune system by vaccination causes a strong microglial reaction in the growing brain, leading to changes in immunological function, resulting in synaptic, dendritic loss, and aberrant appearance pathways. When the microglia are activated, the brain's immune cells release inflammatory cytokines, free radicals, lipid peroxidation products, and two excitotoxins: glutamate and quinolinic acid. Consequently, clinical and pathological characteristics of autism emerge. Microglia are also activated by mercury at quantities of less than 0.5 µg (3 to 5 ng) per gram of moist tissue. High mercuric products are toxic to both the kidneys and the brain. Thimerosal is an organic chemical that includes ethyl mercury and forms a preservative in vaccines. Ethyl mercury hydroxide quickly penetrates the brain and converts to inorganic mercury [6].
Hviid et al. [9] compared children who received immunization with a thimerosal-containing pertussis vaccine to children vaccinated with the same pertussis vaccine formulated without thimerosal and followed them for the symptoms and the signs of autism and other ASD. It was a population-based cohort study to identify the association between thimerosal and autism. In Denmark, they found that the risk of autism and other ASD did not vary greatly between children immunized with a thimerosal-containing vaccine and children immunized with the thimerosal-free vaccine (relative risk (RR) = 0.  [9].
The second hypothesis states that the MMR vaccine can cause autism. Measles is an exceptionally contagious viral infection caused by a paramyxovirus (genus Morbillivirus). It is disseminated through the respiratory system. The clinical features start with a prodrome of flu-like illness, followed by cough, coryza, and conjunctivitis. The measles rash appears as a maculopapular rash on the head that spreads to the torso and arms and legs over three to four days (Koplik spots), which are blue-white plaques on the mucous membranes of the mouth that are pathognomonic. Possible complications include otitis media, viral or bacterial pneumonia, visual loss, acute encephalitis, seizures, and death. Measles is still a significant reason for death and disability in developing countries.
Mumps is a viral infection caused by a paramyxovirus (genus Rubulavirus) transmitted through the respiratory system. Mumps, or measles, virus infection usually results in permanent immunity. Parotitis appears 16-18 days after exposure, and most patients are asymptomatic. Orchitis is more prevalent in postpubertal boys. Mumps complications are rare and most common in adults, including aseptic meningitis, encephalitis, pancreatitis, and deafness [5].
Rubella (German measles) is another viral infection distributed through the respiratory system. It is caused by togavirus (genus Rubivirus). Fever, malaise, upper respiratory symptoms, and a maculopapular rash are the signs and symptoms of rubella, which appear 14 days after contact. Rubella complications are uncommon, although more common in adults and older children. Congenital rubella syndrome (CRS) is a condition that affects the developing fetus and is more severe when infection occurs early in pregnancy. The rubella virus causes fetal death, early birth, deafness, blindness, and severe birth problems, and infection with the virus usually results in lifetime immunity [5].

MMR Vaccine
The MMR vaccine is part of the required childhood vaccination schedule in the United States. It is given in two doses, the first at 12-15 months and the second at 4-6 years. MMR vaccines are live attenuated virus vaccines to prevent measles (rubeola), mumps, and rubella. It is well-tolerated. The most common side effects are injection site responses, fever (5-15%), and a minor rash (5%  (1) rates of ASD are increasing in people who have been given the MMR immunization than in those who have not, (2) a rise in ASD may be occurring as a consequence of the MMR vaccine, (3) the development of ASD is being momentarily linked to receiving the MMR vaccine, and (4) the MMR vaccine may be linked to a new variant form of ASD [4,5].
The tables below will summarize and present the results of these studies; it does not suggest a correlation between ASD and the MMR vaccine [4]. Table 4 shows the characteristic of these studies.  *This is the population studied for analysis data that we extracted; # Same dataset as Peltola et al. [12] was used; + same dataset as Taylor et al. [13] was used. Table 5 shows the comparison of the rate of ASD in vaccinated and unvaccinated Individuals.

TABLE 4: Comparison of the rate of ASD in vaccinated and unvaccinated individuals.
The table shows no statistically significant differences in the rates of autism or ASD between these two populations in adjusted and non-adjusted analyses [4].
ASD: autistic spectrum disorder; RR: relative risk; CI: confidence interval Taylor et al. [13], 1999 Time-series: compared changes in the rates of ASD in periods before and after the MMR vaccine was introduced MMR: introduced in 1987. ASD: no sudden step-up in cases of core and atypical autism in 1987 (p > 0.25); no change in the trend of ASD before and after 1987 Dales et al. [15], 2001 Time-series: compared increasing rates of ASD to rising rates of MMR vaccine coverage (1980-1994 birth cohorts) MMR: increase in coverage from 72% to 82% (14% relative increase). ASD: increase in ASD births from 44/100,000 to 208/100,000 (373% relative increase) Kaye et al.  It did not find an increase in ASD rates in the period of MMR vaccination [4].
ASD: autistic spectrum disorder; MMR: Measles, Mumps, Rubella vaccine; GI: gastrointestinal; OR: odds ratio; CI: confidence interval Table 7 shows the temporal association of ASD with the MMR vaccine. 2022

Analysis Findings
Taylor et al. [13], 1999 Case series: compared ages of ASD diagnosis in those vaccinated before 18 months, after 18 months, and those not vaccinated. Self-matched case series: compared rates of regression, parental concern, or the diagnosis of ASD in specified periods after vaccination There was no significant difference in age at diagnosis among the three groups (p = 0.41). No increased incidence of diagnosis of ASD or regression six months and one year after vaccination; significantly increased risk of parental concern six months after vaccination (p = 0.03) Patja et al. [11], 2000 Case series: identified all reports of vaccine-related complications No cases of post-vaccination ASD (based on passive reporting) in about 1.8 million vaccinees Farrington et al. [14], 2001 Self-matched case series No increased incidence of diagnosis of ASD, regression, or parental concern (24, 36, or 60 months) after vaccination; no increased likelihood of ASD, deterioration, or parental anxiety after immunization compared with before immunization DeWilde et al. [17], 2001 Case-control study Cross-sectional study: compared ages of first parental concern between population samples exposed to MMR vaccine and a pre-MMR vaccine sample. The compared mean interval from MMR to parental anxiety in autistic children with and without regression Mean ages of first parental concern in post-MMR vaccine samples were 19 Case series: determined whether vaccine received before the development of parental concern, after the development of anxiety, or not in autistic children with Gl tract symptoms or developmental regression Gl tract symptoms: 19% received MMR vaccine before parental concern, 15% after concern, and 16% did not receive MMR vaccine (p = 0.48). Regression: 26% received MMR vaccine before parental concern, 26% after concern, and 30% did not receive the vaccine (p = 0.83)   before parental concern, 15% after concern, and 16% did not receive MMR vaccine (p = 0.48). Regression: 26% received MMR vaccine before parental concern, 26% after concern, 30% did not receive MMR vaccine (p = 0.   [2,4,5,7]. The third hypothesis claims that antenatal Tdap vaccination is linked to a higher risk of ASD. Becerra-Culqui et al. examined the link between antenatal tetanus, diphtheria, and acellular pertussis (Tdap) vaccination and the offspring's risk of ASD. With the rise in the frequency of ASD and the increased vaccination in pregnant women, it is more vital than ever to analyze the safety risks associated with prenatal immunization. This study is a retrospective cohort study of mother-baby pairs that gave birth at Kaiser Permanente Southern California hospitals between January 1, 2011, and December 31, 2014. They used digital medical data to get maternal Tdap immunization from pregnancy to the delivery date. The International Classification of Diseases, Ninth, and Tenth Revision codes conveyed an ASD diagnosis. Children were cared for from birth until their first ASD diagnosis, the end of their membership, or their follow-up (June 30, 2017). According to this study, prenatal Tdap immunization was not linked to an increased incidence of ASD. Table 9 shows the frequencies and associations between Tdap vaccination during pregnancy and ASD in infants born between 2011 and 2014.  IPTW: inverse probability of treatment weighting; HR: hazard ratio; CI: confidence interval.
Adjustments were made for the child's birth year, gestational age at birth (<37 or ≥37 weeks); maternal age, race and ethnicity, and education; Medicaid insurance, medical center of delivery, parity, the start of prenatal care, and influenza vaccination during pregnancy [22].

Conclusions
According to our review, there is no link between the development of ASD and immunization. The dramatic increase in the prevalence of ASD created widespread concern. Many theories have been offered to explain the link between vaccination and the development of autism, including changes in immune system function, abnormal organic acid synthesis, mercury toxicity, the effects of gliamorphin on cerebral function, and the link between MMR and autism. However, all these theories remain theoretical, and our review finds no evidence of a link between them and the development of autism. Parents experienced vaccination reluctance following the release of the Wakefield study on the supposed MMR vaccine-autism relationship. It raises concern and challenges vaccine acceptance among parents, leading to the re-emergence of vaccinepreventable diseases. It still raises concern in some parents; we recommend that public health officials continue to advocate and encourage vaccination. The public may require more studies to rule out the association between ASD and vaccination.

Conflicts of interest:
In compliance with the ICMJE uniform disclosure form, all authors declare the following: Payment/services info: All authors have declared that no financial support was received from any organization for the submitted work. Financial relationships: All authors have declared that they have no financial relationships at present or within the previous three years with any organizations that might have an interest in the submitted work. Other relationships: All authors have declared that there are no other relationships or activities that could appear to have influenced the submitted work.