I read that doctors now recommend that the first two be injected and the last two be oral. A child who has received two doses of the oral polio vaccine without reaction faces no additional risk from a third and fourth dose of the oral polio vaccination.
Children usually receive polio vaccinations in four doses by the time they start kindergarten.
OPV contains a weakened version of the live poliovirus that induces immunity. It's taken by mouth in liquid form. IPV contains a chemically killed version of the virus that causes your immune system to recognize and eliminate live poliovirus. It's given by multiple injections. Polio vaccination recommendations recently have changed.