Article by Luis Enjuanes, Isabel Sola and Sonia Zúñiga, researchers at the Coronavirus Laboratory of the National Biotechnology Centre (CNB-CSIC).

Viruses improve their strategy to avoid our immune response, and scientists have to evolve their vaccines to maintain their defence capacity. It is a never-ending struggle, but science has succeeded in eliminating some viruses and may also beable to end SARS-CoV-2, albeit with constant work.

There arecurrently more than 200 vaccine candidates of various types in development to preventinfections by pathogenic coronaviruses such as SARS-CoV-2. These vaccines arebased on a long list of types and technologies.

The pharmaceuticalcompanies involved in their production prefer to develop chemically definedvaccines based on a few elements that cannot evolve, such as purified proteins.However, these vaccines often do not induce strong mucosal immune responses,which is where immunity is required to protect against invasion by respiratoryviruses such as coronaviruses.

Aspirated versus injected vaccines

This immunity isonly efficiently induced by providing an antigen that is expressed in therespiratory mucosa itself, so administration should be intranasal.

This route of administration requires more controls to show that the vaccine does not cross the blood-brain barrier, passing from the blood stream into the brain and causing adverse side effects, which makes it more difficult to be approved by drug regulatory agencies. As a result, pharmaceutical companies initially favour intramuscularly administered vaccines.

However, the intramuscular administration of antigens induces a systemic humoral and cellular immuneresponse - in internal organs - that is low-level and short-lived in the mucosa, which implies the need to administer at least two doses to induce an immune response that provides sterilising immunity - not allowing the virus to grow in a possible subsequent reinfection. This is a common problem with current SARS-CoV vaccines.

‍

Modern vaccines through knowledge

Modern vaccines are those that have been derived from the study of the interaction of the virus with the host. This research has facilitated the identification of genes in the virus that are not essential for its replication, but whose elimination attenuates them.

Usually, these virus genes are involved in neutralising the innate response of the host - the human organism in this case - which acts three to four hours after infection by the viruses, thus preventing the production of interferons, so called because these molecules interfere with virus replication. Attenuated viruses generated by genetic engineering techniques are vaccine candidates.

Other modern vaccines are based on messenger RNAs (mRNAs), which express the S protein ofcoronaviruses.

Based on recombinant or inactivated viruses

A dozen SARS-CoV-2 vaccines have now entered Phase III human clinical trials, the stage where safety and efficacy are robustly verified. Of these, four are based on recombinant adenovirus-derived vectors expressing the SARS-CoV-2 S protein; they are vaccines that are grown to very high titres, so their cost is relatively low.

Two vaccines already approved and currently administered in Spain fall into this category: one from Oxford University and the company AstraZeneca, based on a chimpanzee adenovirus; and a vaccine from the Janssen Pharma, using a vector based on human adenovirus 26.

The others are a vaccine from CanSino Biologics in collaboration with the Institute of Biotechnology in Beijing (China), based on human adenovirus 5; and the Russian vaccinedeveloped by the Gamaleya Institute in Moscow, based on two adenoviruses with serotypes 5 and 26 (one of them is applied in the first immunisation and the other in the second, allowing the immunity induced by the first one not to affect the acceptance of the second dose, also based on an adenovirus).

Other vaccines, such as those of the Chinese firm Sinovac and the China National Pharmaceutical Group Corporation Sinopharm, are based on chemically inactivated SARS-CoV-2. Ninety per cent of people vaccinated with it produced neutralising antibodies specific for the virus, and reported mild adverse symptoms but no major reactions.

Messenger RNA-based

The first two COVID-19 vaccines to be approved are based on mRNAs encoding the Sprotein of SARS-CoV-2. These vaccines are produced by a consortium of thecompanies BioNTech (German) and Pfizer (US), or by the US company Modernaand the US National Health Institutes.

Both induce protection of around 95%, but do not induce sterilising immunity and will need to be improved in the near future.

A dozen vaccines developed in Spain

More than ten vaccines based on some of the above technologies are being developed in Spain. Of these, three are being designed in CSIC laboratories.

The one being developed by Vicente Larraga's team at the CIB-CSIC is based on a synthetic DNA vehicle - a plasmid - in which a gene from the SARS-CoV-2 coronavirus itself is introduced so that, once injected, it stimulates the recipient's immunity.

The vaccine, directed by Mariano Esteban in collaboration with Juan García Arriaza at the National Biotechnology Centre (CNB), uses a variantof the smallpox virus that has been strongly attenuated because around 30% of its genes have been eliminated from its genome.

The vaccine developed by Luis Enjuanes in collaboration with Isabel Sola at the CNB Coronavirus Laboratory is based on RNA replicons derived from the genomes of the MERS-CoV or SARS-CoV-2 viruses. A replicon is a copy of the RNA genome of the virus, from which several geneshave been deleted; it is therefore attenuated and its propagation is poor, which makes it very safe. By maintaining genes encoding antigens that induce protection,these replicons induce a protective immune response.

In preclinical tests in animal models, MERS-CoV-derived RNA replicons have shown sterilising immunity. This prevents the virus from replicating in vaccinated animals, thereby blocking virus transmission.

‍

The goal of a Spanish sterilising vaccine

An analogous technology is being applied to SARS-CoV-2, and two alternative formulations are being developed. One is the in vitro synthesis of the RNA replicon that is administered with polymers for protection.

The second version is based on the production of the replicon RNA inside packaging cells, so called because they provide the proteins needed for mass production of virus-like particles. These vaccines have high potential but haveyet to prove their efficacy and safety in humans.

The immunogenicity and stability of the generated vaccines can be improved by modifying the structure, function, and antigenicity of the SARS-CoV-2 S-glycoprotein. In addition, their genetic information must be updated at least annually so thatthey remain effective against new viral variants that constantly emerge due tothe natural evolution of these viruses. The virus-vaccine evolutionary escalation forces us scientists to be constantly on our guard.

‍

This article is also available in Spanish.