Most COVID-19 vaccines are administered by injection into our upper arm, away from any major blood vessels. Once injected, the muscle cells around the injection site express the spike protein, thereby eliciting an immune response from the body. A large proportion of the remaining dose in the arm drains through our lymphatic system, into the liver and then is destroyed by enzymes there. A very small proportion may ultimately end up in other tissues or the bloodstream. 

Most people get infected by SARS-CoV-2 after inhaling virus-laden droplets from other infected individuals. In this case, the virus first infects cells in the airway, and then deeper in the lungs. Once in the lungs, it then circulates through the bloodstream to the rest of the body. This infection leads to significant damage of the endothelial cells of the lungs. It causes inflammation and inhibits much needed oxygen supply to our blood. 

Extremely low levels of spike protein that circulate from a COVID-19 vaccine are unlikely to have any such effect on the lung endothelial cells. It may in fact offer added protection to vaccinated individuals against more severe symptoms of COVID-19, in the case of a “breakthrough” infection. 

Recent studies conducted in animals have shown that injecting a large concentration of spike proteins alone (not the live virus itself) into the lungs can cause significant damage to these endothelial cells. While these results provide some useful information regarding the nature of COVID-19 infection, they are not directly relevant to side effects in vaccinated individuals. 

Other side effects observed with viral vector vaccines (e.g. AstraZeneca and Johnson & Johnson), including rare blood clots in a small number of recipients, may be related to strong inflammatory responses and reactions to certain preservatives in the vaccines. While this needs further exploration, the European Medical Agency has cleared both vaccines for use in adults since the significant protection against COVID-19 offered by the vaccines outweigh the potential rare side effects in a small number of individuals. 

A recent study conducted on animal hosts showed that injecting a large quantity of the SARS-CoV-2 spike protein (not the live virus) directly into the lungs could cause lung damage. This has been misinterpreted by some who have construed that the results render current COVID-19 vaccines unsafe. 

Separately, there are ongoing investigations into the cases of blood clots observed in a very small proportion of individuals who were administered the AstraZeneca and Johnson & Johnson viral vector vaccines. All vaccines that are currently in use for COVID-19 have undergone a rigorous development and approval process, with systems in place to track adverse events.