About half the US population should be vaccinated² against the H1N1 virus with pregnant women and health workers the top priority, US officials have said.

美国官员称，大约一半美国人口都应该接种H1N1病毒疫苗，特别是孕妇和健康的工人。

The young, pregnant and those exposed to the virus have top priority

A US government advisory³ committee said health officials should prepare to vaccinate¹（预防接种） 160 million people.

The vaccination⁴ campaign, which will involve two doses（剂量，服用量） of vaccine⁵ per person, is due to begin in mid-October.

In the event that not enough vaccine is available, a tighter group of high-risk patients will receive it.

This group also includes people who care for babies, health workers and children between the age of six months and four years.

'Natural immunity⁶'

"The main message is that it's half the population. And it's the younger half of the population, as well as health care workers," said Kathy Neuzil, of the Advisory Committee on Immunisation Practices.

Young adults aged⁷ 19 to 24, and adults - not old people - who have high-risk medical conditions are among a wider group of those to get priority for vaccinations⁸.

Health workers say that old people seem to have higher levels of natural immunity to the virus.

The BBC's Daniel Sandford, in Washington, says swine flu cases are actually decreasing across the US, having peaked 10 weeks ago.

But, our correspondent adds, scientists expect that cases will pick up again during the colder autumn months.

A recent study carried out by the US Centre for Disease Control and Prevention, published on Wednesday, suggests that pregnant women are far more likely to need hospital treatment after contracting H1N1.

They are four times more likely to be hospitalised from swine flu than the general public and can risk complications without speedy anti-viral treatment, the study warned.