Some vaccines, including the Oxford Astra-Zeneca vaccine, are developed using something called a viral vector. A viral vector involves using a weakened and modified version of a virus, in order to teach the body how to activate an immune response against the actual virus. The University of Oxford - AstraZeneca vaccine, AZD122, uses an adenovirus vector as its technology. A spike glycoprotein (S) is found on the surface of the SARS-Cov-2 coronavirus virus through which the virus binds to receptors on human cells (ACE2 receptor) and gains access to insert itself and cause infection. Genetic material of this spike protein is added to ChAdOx1 virus vector so that the adenovirus can stimulate a response from a person's immune system when their body detects it in cells. When the vaccine is injected, it penetrates into the body and gives a blueprint (DNA) for how to defend itself against COVID-19. In this case, that means the cells start to produce club-shaped spike proteins found in coronaviruses, including COVID-19. These three-dimensional spike proteins are so similar to a normal COVID-19 infection that it causes inflammation and the creation of antibodies and T cells. Then, when a vaccinated person is eventually exposed to COVID-19, their body attacks the virus because it already recognizes how to respond to those spike proteins, and can fight against them to prevent infection. Essentially, the vaccine helps train human bodies to detect and eliminate a real COVID-19 infection by showing it mock spike proteins, before COVID-19 can cause any severe symptoms or a severe infection. Similar to all vaccination, it could cause side effects. Completed Phase 3 study results of this vaccine trial are yet to be published, but some early trial data showed that 60% of trial participants reported side effects from the injections. These symptoms included pain, feeling feverish, chills, muscle ache, headache, and malaise and many were treated with paracetamol. Injection-site pain and tenderness were the most common local adverse reactions within48 hours of the injection and a significant number of side effect symptoms were reported in each age group over temporary symptoms of fever, sore throat, headaches or diarrhea. 

There have been several information pieces online falsely claiming that the vaccine injects DNA sequences of various diseases that then make vaccinated individuals succumb to those diseases. Research has suggested that while there could be some residual DNA present when a vaccine is made using cells, the DNA exposure is unlikely to be harmful because the DNA is so fragmented (broken into small parts).  The US Food and Drug Administration limits the size and amount of DNA in vaccines and while we are not aware of any risks associated with DNA fragments that could be present in vaccines, the exact level of risk is not defined.    Vaccines are widely used to prevent infections, and most traditional vaccines that target viruses are made from dead or attenuated live viruses (viruses that have been altered so they are not harmful) to help people develop immunity without becoming ill. Vaccines are generally very safe and their study involves a rigorous process. Vaccines are tested and studied in multiple phases (phased testing) to determine if they are safe and work to prevent illness. Before a vaccine is tested on humans, in the preclinical phase it is tested on laboratory cells or animals. Once it is approved for human research, there are three phases that take place before the vaccine can be considered for approval for public use. During the first stage (Phase I), the new vaccine is provided to small groups of people—the first time the vaccine is tested in humans. The second stage (Phase II) involves testing the vaccine on people who have similar characteristics (such as age and physical health) to the target population, or the group for which the vaccine is intended. The goal of this stage is to identify the most effective dosages and schedule for Phase III trials. The final stage (Phase III) provides the vaccine to thousands of people from the target population to see how safe and effective it is. Once the vaccine has undergone Phase 3 testing, the manufacturer can apply for a license from regulatory authorities (like the FDA in the US) to make the vaccine available for public use. Once approved, the drugmaker will work with national governments and international health organizations to monitor vaccine recipients for potential side effects from the vaccine that were not seen in clinical trials (this is called surveillance). This phase also helps researchers understand how well a vaccine works over a longer time frame and how safe it is for the population.