SARS-CoV-2 vaccines: Lights and shadows

• So far, vaccines to prevent SARS-CoV-2 infection appear as promising approach for curbing the pandemic.

• Vaccines for SARS-CoV-2 are being developed using several different platforms.

• Some concerns have been raised regarding the safety of SARS-CoV-2 vaccines on the basis of rare adverse reactions, particularly thromboembolic.

• The applicability to the real life of efficacy and safety results obtained in Phase III clinical trials requires extensive follow-up in diverse populations.

• Vaccination of individuals with pre-existing immunity may lead to more intense systemic inflammation and might thus trigger unexpectedly intense, albeit very rare, inflammatory and thrombotic reactions.

Vaccines to prevent acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection elicit an immune neutralizing response. Some concerns have been raised regarding the safety of SARS-CoV-2 vaccines, largely based on case-reports of serious thromboembolic events after vaccination. Some mechanisms have been suggested which might explain the adverse cardiovascular reactions to SARS-CoV-2 vaccines. Different vaccine platforms are currently available which include live attenuated vaccines, inactivated vaccines, recombinant protein vaccines, vector vaccines, DNA vaccines and RNA vaccines. Vaccines increase the endogenous synthesis of SARS-CoV-2 Spike proteins from a variety of cells. Once synthetized, the Spike proteins assembled in the cytoplasma migrate to the cell surface and protrude with a native-like conformation. These proteins are recognized by the immune system which rapidly develops an immune response. Such response appears to be quite vigorous in the presence of DNA vaccines which encode viral vectors, as well as in subjects who are immunized because of previous exposure to SARS-CoV-2. The resulting pathological features may resemble those of active coronavirus disease.

The free-floating Spike proteins synthetized by cells targeted by vaccine and destroyed by the immune response circulate in the blood and systematically interact with angiotensin converting enzyme 2 (ACE2) receptors expressed by a variety of cells including platelets, thereby promoting ACE2 internalization and degradation.

These reactions may ultimately lead to platelet aggregation, thrombosis and inflammation mediated by several mechanisms including platelet ACE2 receptors. Whereas Phase III vaccine trials generally excluded participants with previous immunization, vaccination of huge populations in the real life will inevitably include individuals with preexisting immunity. This might lead to excessively enhanced inflammatory and thrombotic reactions in occasional subjects. Further research is urgently needed in this area.