Coronavirus vaccines are designed to provide protection against Covid-19, however, it is possible that an individual who has been vaccinated may become infected again at a later time. The three main reasons are:
Immunity is not gained immediately
It takes time for vaccines to build immunity, and the two authorized coronavirus vaccines, Pfizer / BioNTech and Moderna, require both doses, given several weeks apart, to train the body's immune system. People can be exposed to the coronavirus before or immediately after being vaccinated, so there is no time for the body to develop its defenses. The Centers for Disease Control and Prevention (CDC) says building immunity "usually takes a few weeks".
Vaccines may not offer perfect protection
No vaccine is 100% effective and coronavirus vaccine manufacturers are still evaluating whether doses protect against all infections or only those that cause symptoms. The CDC estimates that 40% of coronavirus infections cause no symptoms. Overall, both vaccines provided about 95% protection during clinical trials - so a small number of people can become infected again after two doses.
Immunity can weaken over time
No one knows how long vaccines will protect against infection. The new coronavirus has been present for a year and the final stages of vaccine testing were completed just a few weeks ago. Pfizer and Moderna followed the volunteers for at least two months after the second doses. The protection provided by vaccines may fade over time and some vaccines require another dose some time later.
There is also the possibility that the new coronavirus will be transformed in a way that makes vaccines less effective. Influenza virus types mutate constantly and this is one reason why people need flu vaccines every year. Doctors hope the coronavirus will not turn into the flu. If this happens, however, the technology used to make the new coronavirus vaccines has been created to adapt easily.
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Source: CNN, Healthline
