Because the first 4 were not effective enough?
The Health Ministry started offering the fifth booster doses of COVID-19 vaccines for a specific group of people, the ministry said Friday.
Individuals who received two doses of the Chinese Sinovac and two doses of the BioNTech vaccine at least three months ago will be able to get an appointment for the fifth dose.
They will be able to receive the Sinovac, BioNTech or the domestically-made Turkovac vaccines as a booster.
Turkovac, a domestically developed inactive COVID-19 vaccine, is finally available to the public after receiving emergency use approval.
https://www.dailysabah.com/turkey/turkey-starts-offering-5th-dose-of-covid-19-booster-shots/news