It came after more than 1000 people contracted mumps in Auckland this year, most of them aged between 11 and 29, with low rates of measles, mumps and rubella (MMR) vaccination.
Mr O'Connor said all the Whanganui cases fitted that age bracket of what has been called the "lost generation".
"There has been a great improvement in getting people immunised in the last 15 years but rates before then were lower and some did miss out so they could be more susceptible."
Children have this vaccine at 15 months and four years.
He said everyone was entitled to two doses of MMR so people who may have missed out could still get immunised.
"We are just concentrating on maintaining a good vaccination programme for young children," Mr O'Connor said.
The ministry has responded to the letter - jointly signed by the chief executives of Auckland, Counties Manukau and Waitemata district health boards, and two senior Auckland Regional Public Health service officials - offering extra vaccinations early next year, RNZ reports.