evidence: There are also reports of people who have been fully vaccinated getting sick with COVID-19. This is expected – no vaccine is 100% effective against all forms of the illness it's designed to protect against. People who had weakened immune systems before they got vaccinated may be especially prone to a post-vaccination case of COVID-19 if they're exposed to an infected person during their contagious period. [...] Just as with other vaccines, the COVID-19 vaccines can cause temporary effects soon after they enter the body and start teaching the immune system to go after the coronavirus. The most common reactions are headaches, arm pain, body aches, chills or fever lasting a few hours to a few days. Taking an over-the-counter painkiller can help ease these. The CDC has more information about what to expect and what to do if you experience a vaccine reaction. [...] People with weakened immune systems may be less protected by a COVID-19 vaccine than someone with a healthy immune system, but they are still encouraged to get vaccinated.