For the first time, human attention and hopes are all focused on the creation of a vaccine. In fact, only when we will be able to prevent the infection caused by the coronavirus SARS-CoV-2, will we be able to return to a normal life, free from masks and movement restrictions.

At regular intervals we read about new vaccines being introduced in various clinical trials. But how many coronavirus vaccines are in development? Currently (October 2020) about 180. The various laboratories have adopted all known development methodologies to obtain at least one candidate who will be successful in the clinic. Let’s now see what types of vaccines exist:

Inactivated vaccines

This is the most traditional type of vaccine. The virus is grown in cell cultures, then chemically inactivated (read: killed) with formaldehyde or BPL (β-Propiolactone). Six inactivated virus vaccines are currently on the market: against poliovirus, the hepatitis A virus, Japanese encephalitis, tick-borne encephalitis, rabies and influenza. They are generally administered intramuscularly and can be adjuvanted with alum or other adjuvants. Because with this type of vaccine the whole virus is presented to the immune system, it is likely that, in the case of SARS-CoV-2, the immune response interests not only the spike protein, the favorite target, but also the matrix, the envelope and nucleoprotein. Inactivated vaccines can be produced relatively easily, but the yield is limited by the reduced amount of virus collected from cell cultures and the need for production facilities with Biosafety Level 3 (BSL3). Several inactivated coronavirus vaccines are currently in clinical trials, with three Chinese candidates in phase III and one Indian, one Kazakh and one Chinese candidate in phase I/II.

Live attenuated vaccines

An attenuated vaccine is developed by reducing the virulence of the pathogen; the virus therefore remains able to replicate after administration, but not sufficiently to cause disease in the recipient. However, it will induce an immune response that can protect against future real infection before the virus is cleared from the body. Examples of attenuated vaccines are against measles, mumps, rubella, flu, chicken pox, rotavirus, yellow fever, etc. Since the virus is still alive and able to replicate, there is a remote risk of its return to virulence after administration. Despite this potential risk, live attenuated virus vaccines offer significant advantages over inactivated vaccines, in fact: (i) they activate a wide range of immune responses, (ii) they induce rapid and long-lasting immunity, (iii) often reduce need for booster vaccinations, (iv) do not require adjuvants, (v) can be produced at relatively low cost, and (vi) can be administered orally/nasally. Regarding SARS-CoV-2, the advantage of being able to administer the vaccine nasally would ensure a mucosal immune response that would protect the upper respiratory tract, the main gateway for the virus. Only three live attenuated coronavirus vaccines are currently in the preclinical stage.

Recombinant protein vaccines

The structure of SARS-CoV-2 virus.

This type of vaccine is more recent. While the previous categories expose the entire virus (dead or attenuated) to the immune system, recombinant protein vaccines only expose the virus antigens, to better stimulate the immune system. Indeed, an antigen is an element of a pathogen (virus or bacterium), capable of inducing an immune response. Although this strategy makes vaccines safer (reducing side effects) and easier to produce, it often requires the incorporation of adjuvants to elicit a strong immune response, because antigens alone cannot ensure long-term immunity. Examples of vaccines based on recombinant proteins are against pertussis, hepatitis B and papillomavirus. Regarding SARS-CoV-2, there are three types of vaccines based on as many recombinant proteins: i) the whole spike protein (S), ii) the RBD region of the spike protein, which is the one responsible for binding to the ACE2 receptor on human cells and iii) virus-like particles (VLPs) which can be defined as “empty boxes” of the virus, with no genetic material inside but which expose the virus proteins on their surface. Alongside the advantage of reducing side effects and not having to interact with the live virus during vaccine preparation, there are some disadvantages: i) the spike protein is difficult to produce in large quantities, becoming a limiting factor; ii) the RBD region is instead easier to produce, but very small and therefore offers less antigenic variety than the whole spike protein; iii) VLPs are difficult to produce. Many vaccines of this type are currently in preclinical development, and several S and RBD vaccines have entered clinical trials (along with only one VLP candidate). Like inactivated vaccines, recombinant protein-based vaccines are also administered by intramuscular injection, therefore they should not guarantee strong immunity to the mucosa of the upper respiratory tract.

Vaccines exploiting replication active/inactive vectors

This type of vaccine uses well-known and non-dangerous viral vectors (capable of replicating in the body, or modified so that they cannot replicate), containing the gene that codes for an antigen against which an immune response should be induced. In the case of SARS-CoV-2, the inserted gene encodes for the spike protein. If the vector is able to replicate, the immune response will be stronger (as in the case of attenuated vaccines), while if replication is inhibited, the vector will be safer, but less effective. Currently there are only two candidates of recombinant vectors capable of replication in the clinical phase, while several candidates (both capable of replication and non-replicating) are in the preclinical development stage.

DNA/RNA vaccines

DNA and RNA are the two forms of the genetic material. Therefore, to produce an antigen of interest, first it must be encoded as genetic information. This strategy is followed by DNA or RNA vaccines: instead of the antigen (in our case the spike protein of the SARS-CoV-2 virus), its related genetic material is administered as vaccine to the body, so that the spike protein is produced directly by the cells of the host organism, causing an effective immune response. It is important to underline that this genetic material is not integrated in the human genome, which therefore is not altered following vaccination. This technology allows the production of vaccines in large quantities, but being of recent development, it is not yet known whether there is a risk about long-term stability (i.e.: storage and large-scale distribution), while there are no doubts about its safety for health. Currently, 4 DNA vaccines (and none RNA) are in clinical phase I/II.

The production strategy of SARS-CoV-2 vaccines

I would like to spend the last few words to describe how a vaccine is generally produced, and how differs from the production of the one against SARS-CoV-2.

Standard strategy

The traditional methodology can take up to 15 years (or more) and begins with a long research phase during which vaccines are designed and exploratory preclinical experiments conducted (it takes several years). This phase is usually followed by one in which more accurate preclinical experiments and toxicological studies (on animals) are performed and in which large-scale production processes are defined (2-4 years). During this process, an application is made for an investigational new drug (IND), which is followed by phase I (1-2 years), II (2 years) and III (2-3 years) of clinical trials. Once the results of the phase III studies are available and if they meet the pre-established objectives, a biologic license application (BLA) is submitted, which is then reviewed by regulatory agencies (1-2 years) before authorizing the sale of the vaccine. Only then does large-scale production begin.

Accelerated SARS-CoV-2 strategy

Thanks to the knowledge gained during the development of the SARS-CoV-1 and MERS-CoV vaccines, it was possible to skip the research phase. With the knowledge acquired, phase I/II of clinical trials were launched simultaneously. The phase III studies were initiated after interim analysis of phase I/II results, keeping several phases of the clinical trial running in parallel. Meanwhile, vaccine manufacturers have begun – at their own risk – large-scale production of potential vaccines, without waiting for the results of clinical phase III. The exact path to obtain the sales license is not clear yet.