I mean, we're arguing semantics here. Getting a shot then 3 years later getting another shot (or for polio, a total of 4 doses) can be called whatever you want. Kind of like how Pfizer is calling their 3rd shot a booster and Moderna is just calling it the 3rd dose in the series. I'm sure you got one measels shot as an infant and another as a young tot. Call it two doses, call it a booster. Same rose.
And, luckily for us, measles and polio are diseases that once immune, you stay immune for life. Not so for the Tetanus, diptheria, pertussis vaccine - that one has recommended boosters every 5 to 10 years throughout adulthood, because immunity wanes and if you don't stay up to date, you're vulnerable (and with Pertussis, at risk of transmitting it to infants for whom it's very dangerous).