The availability of CHO vaccine for the first two doses of Vero vaccine and the third dose of CHO vaccine will depend on the availability of a booster in your area and your situation. The Vero vaccine is usually an inactivated vaccine and the CHO vaccine is usually a recombinant protein subunit vaccine. If inactivated vaccine was given in the first two doses, a booster dose of inactivated NeoCrown vaccine is now recommended in most areas. In some areas, sequential booster vaccination has been introduced, whereby a different type of CHO vaccine is selected for the booster shot than for the basic program. In principle, sequential vaccination is possible and promotes better immunization of the body.