INTRODUCTION

Inactivated Vaccines

Inactivated vaccine production is a more traditional method. Many inactivated vaccines have been developed for various viruses, such as influenza, hepatitis A, and poliovirus (1). Inactivated vaccines contain killed pathogens through physical or chemical processes; therefore, they cannot cause disease and are considered safe (2). Adjuvants are used to strengthen the immune response to vaccines (3), and their storage and transportation conditions are easier to achieve than in those of other vaccine types. They can be stored at 2 °C-8 °C. However, there are some disadvantages: i) Large amounts of virus or bacteria are needed to produce vaccines; ii) during the inactivation process, the immunogenicity of the pathogen can be affected; and iii) booster doses are generally needed to produce adequate immune response (2,4). Since the start of the coronavirus disease-2019 (COVID-19) pandemic, nine inactivated vaccines have been authorized for emergency use by many countries, so far, including CoronaVac developed by Sinovac Biotech (China), BBIBP-CorV by Sinopharm (China), and BBV152 (Covaxin) developed by Bharat Biotech (India) (5). Only two of them were approved by the World Health Organization (WHO), i.e., CoronaVac and BBIBP-CorV, as of August 2021 (6).

Phase III clinical trials of CoronaVac were conducted in China, Brazil, Turkey, Chile, Indonesia, Philippines, and Hong Kong (6), and some of them are still ongoing. The Brazilian trial demonstrated an efficacy of 51% [95% confidence interval (CI) 36-62] against symptomatic disease, whereas the Indonesian and Turkish trials presented 65% (95% CI 20-85) and 84% (95% CI 65-92), respectively. By contrast, the efficacy of vaccines against hospitalization was 100% in Turkey (95% CI 20-100) and Brazil (95% CI 56-100). In Chile, a cohort study of more than 10 million individuals was conducted. They were followed for 2 months after vaccination, and vaccine effectiveness was assessed. The efficacy rates against symptomatic COVID-19, hospitalization, intensive care unit admission, and death were 67% (95% CI 65-69), 85% (95% CI 83-97), 89% (95% CI 84-92), and 80% (95% CI 73-86), respectively (7-9). On June 1, 2021, the WHO validated the vaccine for emergency use, and 39 countries including Turkey were granted emergency use of authorized vaccines as of August 12, 2021. The efficacy of BBIBP-CorV (Sinopharm) was evaluated in the COVIV-02 study. It is being conducted in Bahrain, Egypt, United Arab Emirates, and Jordan (clinicaltrials.gov, NCT04510207). Interim results showed an efficacy of 78.1% (95% CI 64.8-86.3%) against non-severe COVID-19. The WHO approved BBIBP-CorV for use on May 7, 2021, and it is being used in 59 countries as of August 12, 2021 (7). BBV152 (Covaxin) vaccine demonstrated an efficacy of 77.8% (95% CI 65.2-86.4) against symptomatic disease and 93.4% (57.1-99.8) against severe disease in a phase 3 clinical trial in India (10). This vaccine has been approved in nine countries, including Guyana, India, Iran, Mauritius, Mexico, Nepal, Paraguay, Philippines, and Zimbabwe, as of August 2021.

Variants of concern: A study revealed that serum samples obtained from individuals who received two doses of CoronaVac showed nearly the same neutralization activity against alpha (B.1.1.7) variant but significantly reduced activity against beta (B1.351) and gamma (P.1) variants (11). In another study, Wang et al. (12) revealed similar results for BBIBP-CorV vaccine against alpha and beta variants.

A report evaluating the efficacy of BBV152 showed a decreased neutralization activity against beta, gamma, delta (B.1.617.2), and kappa (B.1.617.1) variants, whereas the activity was quite similar against the alpha variant. In the same study, vaccine effectiveness was 65.2% (95% CI 33.1-83.0) against the delta variant (10). Further studies are warranted to get a better understanding of the efficacy of inactivated vaccines against new variants.

Nucleic Acid Vaccines

Nucleic acid vaccines are DNA and mRNA vaccines. DNA vaccines enable the expression of target proteins in the individual who was vaccinated. These vaccines consist of plasmid DNA containing the gene of the target protein and mammalian expression promoter regions. Its routine usage has been avoided because of drawbacks, such as low immunogenicity, reliance on mRNA for function, and possibility of being mutagenic by interacting with the recipient DNA (13). Although mRNA vaccines were originally utilized during the COVID-19 pandemic, the first studies on these vaccinations were conducted in 1989. The ability of mRNA packaged in a cationic liposomal nanoparticle (LNP) to enter multiple eukaryotic cells was first demonstrated; subsequently, when naked (unprotected) mRNA was injected into mouse muscle cells, it remained stable for several days and generated target antigens (14). These findings show that using an in vitro transcription mRNA molecule rather than plasmid DNA to enable the production of a targeted protein in living tissue without the use of a viral carrier is safer (vector) (14,15). Because the mRNA molecule acts in the cytoplasm rather than in the nucleus, there is no risk of insertional mutagenesis when it joins the host genome, and its structure can be destroyed by normal cellular activity; thus, its half-life can be controlled by various chemical modifications (16).

One of the most significant advantages of the molecule is that it can be turned into a more effective molecule by modifying it chemically to make it more stable and translatable (17). It is not dependent on a viral vector for in vivo activity and does not induce side effects caused by antivector immunization as it allows rapid uptake and expression into the cytoplasm with carrier molecules (18,19). Self-amplifying and self-replicating vaccines and conventional (non-replicating) vaccines are two types of MRNA vaccines. RNA-dependent RNA polymerases are commonly used in self-replicating mRNA vaccines to promote mRNA amplification and target antigen expression. Long-term antigen production is guaranteed in this manner; however, the vaccine has a huge molecular structure because it comprises numerous promoter regions and necessitates transport by viral vector (20).

In contrast, traditional mRNA vaccines are synthetic molecules made in vitro from plasmid DNA and bacteriophage RNA polymerase (15,16) The created unprotected mRNA molecules must be packed with lipid nanoparticle carriers so that they can enter the target cells via endocytosis. LNPs also shield these compounds from enzymatic degradation (21,22). Furthermore, lipid nanoparticles enable the regulated distribution of mRNA molecules in the body, allowing them to reach their target cells (16).

To induce immunogenicity, mRNA vaccines must first reach the ribosomes and ensure the production of target antigens. After this target intracellular endogenous antigen is synthesized, it is presented to CD8+ T-cells by major histocompatibility complex (MHC) class I molecules, which sensitize them. Additionally, antigen-presenting cells use MHC class II molecules to activate CD4+ T lymphocytes. This is very important in the induction of humoral immunity as well as cellular immunity. In addition, antigen-presenting cells transport the antigens to the lymph nodes, where they are transferred to non-sensitive B-cells, triggering the immunological process that leads to antibody production. Memory B-cells are activated when a vulnerable individual is exposed to target antigens again. In conclusion, mRNA vaccines provide cellular and humoral immunity by activating CD4+, CD8+ T-cells, and B lymphocytes (23-25). According to the WHO, 184 vaccine candidates are in preclinical development and 112 are in clinical trials for COVID-19, with 11 and 18 being DNA and RNA vaccines, respectively (26). Of these vaccines, Pfizer/BioNTech BNT162b2 has been licensed by the Food and Drug Administration (FDA) for use in individuals aged >16 years and has been approved for emergency use for individuals aged 12-16 years (27). Moderna’s mRNA-1.273 vaccine also has been licensed for emergency use to prevent COVID-19 (28).

 BNT162b1 and BNT162b2 mRNA vaccines were created by Pfizer/BioNTech. The severe acute respiratory syndrome-coronavirus 2 (SARS-CoV-2) spike protein is encoded in both vaccines, which are packed with LNP and undergo nucleotide modification. In phase I/II trials, both vaccines were proven to be efficacious and safe in prime and boost vaccine regimens, and BNT162b2 was able to advance to phase II/III studies (NCT04368728) because it has milder adverse effects (29). In a phase III study including 43,000 patients aged ≥16 years, the case group received 30 µg of the vaccine, and the control group received the same quantity of saline solution at 3-week interval. Vaccine efficacy was measured 7 days after the second dose, with rates of 95% and 94.6% in those without and with a history of COVID-19, respectively. The findings of this investigation show that BNT162b2 is efficacious and safe against COVID-19 (30). The vaccine’s efficacy against symptomatic disease declined with time, from 90% at the end of the second month to 84% at the end of the sixth month, according to an unpublished follow-up report that included phase III participants (31). Although the vaccine’s neutralizing antibody levels were lower against beta and delta variants, investigations have indicated that it maintains neutralizing effectiveness against variants of concern (32,33). The BNT162b2 vaccination should be given in two intramuscular doses of 0.3 mL each at an interval of 3 weeks. The time between the two doses can be extended up to 42 days, but not less than 3 weeks. If the vaccine was not administered at this time, the schedule should be re-established (34,35). Because most studies involved two doses, the protective effectiveness and durability of a single dose are uncertain. Furthermore, a single-dose vaccine was found to be ineffective against the alpha, beta, and delta forms (36,37). In immunocompromised groups, such as recipients of chemotherapy, patients with hematological malignancies, recipients of hematopoietic stem cell and solid-organ transplantation, patients with human immunodeficiency virus (HIV), and others, a third dose has been advised, provided that it is given at least 28 days after the second dose (38). Local adverse effects such as redness, swelling, and pain at the injection site are common after the second dosage, whereas serious postvaccine reactions are rare. The majority of these reactions are mild and generally resolve within 2 days.

Fever, fatigue, headache, arthralgia, and myalgia are other symptoms that have been described (39). Anaphylaxis following vaccination was observed in five cases per million doses (40). Although Bell’s palsy occurred during phase III clinical trials, no link between immunization and Bell’s palsy was discovered (41). The surveillance system found no thrombosis-related events connected with mRNA vaccinations (42). Moderna’s mRNA-1273 is another mRNA vaccination that has been licensed by the FDA for emergency use. The recommendation was to take two doses at 28 days apart. It contains the target antigen and spike protein lnp packed with the mRNA molecule (43).

Individuals aged 18-55, as well as those aged >55 years were studied in a phase I clinical trial. Despite the decline in CD8+ T-cell response, considerable CD4+ T-cell response was seen in both the groups, and they had comparable and sufficient effectiveness and safety profiles (44,45). In a phase III clinical trial, vaccination activity was shown to be 94.1% effective (14 days after the second dose) in avoiding symptomatic illness; however, adults aged >65 years accounted for 86.4% of this activity. This vaccine has also been studied with two doses, and the efficiency and safety of a single dosage are unknown (46). The side effects are comparable with those of BNT162b2 and are usually local reactions including pain, redness, and swelling at the injection site, which usually resolve within 2 days of vaccination. Fever, fatigue, headache, arthralgia, and myalgia are other symptoms that have been described. Although adverse effects are less likely to occur in individuals aged >65 years, they have been considered in this vaccine (47). Anaphylaxis was found in 2.8 cases per million doses after vaccination. A history of allergy was noted in 86% of these cases, and 90% of allergies occurred within half an hour after vaccination. In the United States, some cases of Bell’s palsy are assumed to be related to the vaccine, but no link has been identified between vaccination and Bell’s palsy because the rate is lower than the general population (48,49). In the United States, a case of postvaccine sinus vein thrombosis with thrombocytopenia was recorded; however, it was unclear if the link was accidental or causal and has been reported as an extremely rare side effect (49-51).

Viral Vector Vaccines

Although the traditional approach is being used when developing a vaccine to control the COVID-19 pandemic, viral vector vaccines have also been used based on molecular techniques. The basic characteristic of viral vector vaccines is that a genetic piece of the virus is transferred into another virus, and immunity is produced following administration of a vaccine to the body (52). In vaccines obtained through this method, an immune response resembling a real viral infection occurs (53). The characteristics of the carrier virus of a known structure determine the side effects and production details of the vaccine. Thus, vaccine studies against a new and unknown pathogen can be conducted more securely. Replicable (attenuated) viruses and non-replicating (inactivated) viruses are used as carriers in these vaccines (53). Although vesicular stomatitis virus is most frequently used as a replicating carrier virus, adenoviruses are most often preferred as the non-replicating carrier virus (54). At present, carrier viruses are used in the vaccine production studies for Ebola, hepatitis C, influenza, tuberculosis, and HIV (55). By activating both cellular and humoral immunity, these vaccines produce long-term immunity. They are advantageous because they can be produced easily, can be applied to the mucosa, and are of low cost (56). In contrast, COVID-19 viral vector vaccines are based on the transfer of genes encoding the spike protein of SARS-CoV-2 via adenovirus. Although this technology was used in 17 vaccine candidates, only three adenoviral vector vaccines have been approved for use (57).

Oxford/AstraZeneca Vaccine: At the beginning of the pandemic, a viral vector vaccine, which is a gene technology product developed by transferring the genes encoding the spike proteins of SARS-CoV-2 into the non-replicating chimpanzee adenovirus, was produced in collaboration with the Oxford University and AstraZeneca (58,59). In the preclinical trials of the vaccine, high antibody levels were observed in pigs (8-59). Phase I/II studies of the vaccine, codenamed AZD1222, were conducted in England and South Africa, phase II/III studies were performed in England, and phase III studies were conducted in Brazil. A total of 23,848 volunteers aged >18 (7,548 from England and 4,088 from Brazil) participated in the four blinded, randomized controlled phase III study conducted in these three countries. In these studies, wherein vaccine efficacy was assessed based on symptomatic disease and polymerase chain reaction test, the vaccine was found to be effective against symptomatic COVID-19. As a result of the phase III trials, the efficacy rate was 62.1% in the group that received two full doses of vaccine, whereas it was 90% in the group that received half dose in the first dose and full dose at the second dose. Two doses of the vaccine are administered intramuscularly at 28 days apart (59). In the phase I/II study conducted with 1077 volunteers in England, the most common side effects associated with the vaccine were pain, fever, chills (shaking), muscle pain, headache, and weakness (58,59). Similar adverse effects were reported in phase II/III studies conducted in England, in two different centers with 560 patients, and no serious side effects were observed once again (58). Transverse myelitis developed in two individuals in the group that received two doses of the Oxford/AstraZeneca vaccine and in one individual in the control group that received meningococcal vaccine; however, no direct association with the vaccine was found (60).

Sputnik V (Gam-COVID-Vac): The Sputnik V vaccine, one of the first registered COVID-19 vaccines, was developed in Russia by the Gamaleya Institute. It is a vector vaccine developed by transferring the genes encoding the spike proteins of the SARS-CoV-2 genome into adenovirus type 26 at the first dose and adenovirus type 5 at the second dose. It was named Gam-COVID-Vac by Russia in August 2020. Sputnik V has now reached 61 countries as of December 2020, including Russia, Argentina, Belarus, Hungary, Serbia, and United Arab Emirates (61). Sputnik V, which was recognized as safe based on the results of phase I and II clinical trials, produced a high cellular and humoral immune response. Following that, in a double-blind randomized placebo-controlled study conducted in 25 clinics in Russia, 21,977 volunteers aged ≥18 years were assigned to the vaccinated group, whereas 5476 volunteers were assigned to the placebo group. The vaccine group received two doses of the vaccine at 21 days apart. The presence of COVID-19 at least 21 days following the first vaccination was assessed. COVID-19 was detected in 16 individuals (0.1%, n=14964) in the vaccinated group and in 62 individuals (1.3%; n=4902) in the placebo group; thus, its vaccine efficacy was 91.6%. The vaccine was 100% effective in preventing severe COVID-19 (62). Although mild and moderate local side effects were observed, no serious side effects were detected (62).

Johnson & Johnson (Ad26.COV2.S): This is a vaccine developed by delivering genes encoding spike proteins of SARS-CoV-2 virus via adenovirus type 26. It is the third COVID-19 vaccine to be approved for emergency use in the United States. The vaccine is administered intramuscularly as a single dose in individuals aged >18 years (63) Adverse effects and immune response were compared with the placebo group in a phase I/II trial wherein 402 volunteers aged 18-55 and >55 years were separated into two distinct cohorts. Although the most common local adverse effect is pain at the injection site, the most common systemic adverse effects were weakness, headache, and muscle pain. Neutralizing antibodies were detected in 90% of the participants 29 days after the first dose and 100% of them on day 57 (64). Phase III trials were conducted with 40,000 volunteers aged 18-100 years. Protective rates against symptomatic COVID-19 14 and 28 days after vaccination were 66.3% and 65.5%, respectively. The efficacy observed after 14 days varied by gender, age, race, and ethnic groups. The highest efficacy was 77% in the United States. Although some hospitalizations were associated with COVID-19 in 31 participants 14 days after the vaccination, 2 of them were in the vaccinated group, and no hospitalizations were noted in the vaccinated group 28 days after vaccination. The Janssen COVID-19 vaccine may be also protective against asymptomatic COVID-19 (63). Thrombocytopenia and cerebral venous sinus thrombosis were reported in 42 women aged 18-49 years after receiving the Janssen vaccine for 1-2 weeks (60). The Centers for Disease Control and Prevention and FDA have warned against the risk of vaccine-induced thrombocytopenia and thrombosis, particularly in women aged 18-49 years (65).

Protein Subunit Vaccines

These vaccine types produce immunity by containing only the protein part of the virus without including the genetic material of the virus. Hepatitis B and pneumococcal vaccines are examples of protein subunit vaccines (66). There exist 38 vaccine candidates that are intended to produce immunity by transferring SARS-CoV-2 proteins to the human body using this method.

Novovax: Novavax, which is the first protein subunit vaccine to complete its phase III trial, was found to produce 89.7% protection in the original strain, as a result of a study conducted at 33 centers in the United Kingdom. Ten participants in the vaccinated group and 96 participants in the placebo group developed COVID-19 7 days after the second dose. Hospitalization and death were not observed in any of the 10 individuals who received the vaccine and subsequently became infected. Although a small number of mild and moderate adverse effects were observed, no serious adverse effects were detected (67).

Table 1 compares the characteristics of viral vector vaccines and protein subunit vaccines (67).

CONCLUSION

COVID-19 vaccines granted for EUA so far, are safe and effective. Vaccines are the most important tools in protection from the disease.

Development of new vaccines is still ongoing.

Ethics

Peer-review: Internally peer-reviewed.

Authorship Contributions

Concept: S.A., H.R.S., M.R.D., S.Ar., F.Ş., Design: S.A., H.R.S., M.R.D., S.Ar., F.Ş., Data Collection or Processing: S.A., H.R.S., M.R.D., S.Ar., F.Ş., Analysis or Interpretation: S.Ar., F.Ş., Literature Search: S.A., H.R.S., M.R.D., S.Ar., F.Ş., Writing: S.A., H.R.S., M.R.D., S.Ar., F.Ş.

Conflict of Interest: No conflict of interest was declared by the authors.

Financial Disclosure: The authors declared that this study received no financial support.