A breakthrough case is defined as involving a person who has SARS-CoV-2 RNA or antigen detected on a respiratory specimen collected ≥14 days after completing the primary series of an authorised COVID-19 vaccine.
Since vaccines require about two weeks to become effective, a person is not considered fully vaccinated until two weeks after they completed the recommended number of doses for the vaccine they received. A person who tests positive for COVID-19 between their first and second doses of two-dose vaccines, or a person who tests positive before the two-week period after their final dose, is not considered a breakthrough case.
No vaccine provides 100 percent protection against infection, so a fully vaccinated person can later get the disease they were vaccinated for. Breakthrough cases are not new, and not unique, to COVID-19.
Generally, COVID-19 vaccines are very effective in preventing mild symptoms, severe disease, and hospitalisations due to COVID-19 disease. But it is still possible for a fully vaccinated person to get infected if they are exposed to the virus.
Fully vaccinated people usually get some immunity against the disease. Therefore people with a vaccine breakthrough infection usually have either no symptoms or mild symptoms. Typically, breakthrough cases occur when an individual has had a lower immune response from the vaccine.
Though these individuals have no symptoms, yet they can still spread the virus to others. That is the reason why even after getting vaccinated for COVID-19, people still need to follow all of the recommended quarantine procedures such as wearing a mask, distance keeping, avoiding crowded settings, etc to help to prevent exposure to the virus and unknowingly spreading it to others.