New Zealanders are being urged to have flu jabs soon, to give the country the best chance of a smooth ride through a likely second round of the international swine flu pandemic.

The Ministry of Health predicts the resurgence may begin as soon as the end of this month, so it wants people to talk to their doctor or nurse now about vaccination. It takes up to two weeks to develop immunity after vaccination.

The second wave is expected so soon because of the experience of the Northern Hemisphere late last year in its autumn and winter.

"The indications from the Northern Hemisphere are that if we are going to get a second wave, it's likely to start quite early in autumn - in March or April," said the ministry's director of public health, Dr Mark Jacobs.

The injection, licensed for use on those over 6 months old, is state-funded for the elderly, young children at high health risk and people with any of a range of listed health conditions. The programme runs until the end of June.

Others have to pay about $20, although many employers pay for their staff to be vaccinated.

The first wave of the pandemic hit New Zealand last April with the arrival of travellers returning from Mexico, the first country affected. At first it caused widespread fears and attracted intense coverage because no one knew if it would be as severe as the 1918 Spanish flu, or, as it turned out, something generally much milder.

The virus, a new strain now called H1N1 2009, generally caused a relatively mild disease for those who experienced symptoms, although for some it was a serious illness and fatal in a few cases.

No one knows how many people in New Zealand were exposed to the virus although researchers, who are taking blood samples from people to look for antibody evidence of swine flu exposure, expect to come up with a reliable calculation.

During the first wave, the ministry calculated that up to half the population could have caught the virus.

A total of 3200 cases were reported, but the actual number is likely to be far higher because once the virus had become widespread, health authorities stopped trying to confirm every case by laboratory testing and encouraged otherwise healthy people experiencing relatively mild symptoms to look after themselves at home.

Swine flu put 1014 people in hospital and was confirmed as the cause of 20 deaths, although a number of other deaths are still being investigated.

An unknown but large number of people would have been exposed to the virus without experiencing any symptoms. This bodes well for a mild second wave because they will be immune to the virus.

Dr Jacobs said that was the ministry was relying on this natural immunity - and advocating rapid uptake of the vaccine - in its hopes that any second wave would have only a mild impact.