Almost every medication or vaccine includes some side effects. Many people who receive flu shots or even shingles vaccines are likely to have sore arms and some redness where they received the jab. The second dose of some COVID-19 vaccines can cause common side effects including muscle aches, fever, fatigue, diarrhea, nausea and more.

Some COVID-19 vaccines have been potentially linked to extremely rare but severe side effects. AstraZeneca and Johnson & Johnson vaccines may be potentially linked with very rare forms of blood clots, some of which could lead to the possibility of deaths. However, there has been no conclusive link between the vaccines and the blood clots. The United Nations noted that there is a "plausible" connection between the AstraZeneca vaccine and blood clots but the World Health Organization is still investigating this issue.

National and international drug regulation agencies are evaluating whether or not these vaccines can cause or be linked to blood clots. In the meantime it is important to note that people infected with COVID-19 are 100 times more likely to develop blood clotting than people who were not infected.

The risk of getting a blood clot after a COVID-19 infection is shown to be eight times higher than the risk of a blood clot after the AstraZeneca COVID-19 vaccine. Blood clots in the general population are relatively common so it's important to remember that though these might be rarely associated with vaccines, they are happening in vaccinated people at a rate that is much lower than naturally occurs in large populations.

Other vaccines have less peer-reviewed data to evaluate. However, the Sputnik V vaccine, CoronaVac and Covaxin have not been associated with excess mortality in studies as of April 21, 2021. To be sure, early data showed some reports of serious side effects after first shots of both the CoronaVac and AstraZeneca vaccines in the Philippines. These reports were rare — 137 out of 240,000 administered in the country.

The Pfizer-BioNTech vaccine has been shown to be safe and is not linked to deaths or widespread severe side effects in several scientific journals. It is challenging for doctors and researchers to confirm that a vaccine directly led to a person's death, especially as such large numbers of people are getting vaccinated.

Until more data is available for all vaccines over a longer period of time, conclusive evidence about their potential side effects is not available. Each country's national regulatory agencies determine whether or not they believe each vaccine is safe and effective for their populations. If a COVID-19 vaccine could directly lead to death, it is highly unlikely regulatory agencies would approve their use.

As more people receive COVID-19 vaccines around the world, the likelihood of seeing more side effects linked to them is high. There is no way clinical trials can capture all the potential responses each person will have to the shots. Each country's health regulatory agencies, the World Health Organization, and other international bodies evaluate whether or not a vaccine is safe for wide distribution among millions of people. If these organizations are concerned about potential side effects or deaths, they generally pause vaccine rollouts until they can research the cause of those issues.