4. In the case of every child residing in Singapore who has received primary diphtheria vaccination, booster diphtheria vaccination consisting of one injection shall be compulsory —
(a)
12 months after the primary diphtheria vaccination; and
(b)
within 12 months after the child has attained the age of 6 years,
except that paragraph (b) shall not apply if the child has already received the first booster diphtheria vaccination within a period of not more than two years before attaining the age of 6 years.