The federal government last year declared a target for full vaccination to be available by the end of October.

Health Minister Greg Hunt last month said people would be offered at least a first dose after new evidence emerged about the AstraZeneca vaccine, which is more effective with a 12-week gap between jabs.

Health Department secretary Brendan Murphy said it was unlikely the government’s original goal could be met.

“We don’t know whether we’ll be able to achieve two shots by October,” he told the Senate’s coronavirus response committee on Thursday.

“All I’m saying is that with a 12-week interval it’s going to be difficult.”

The original modelling behind the October completion date was based on a four-week break between injections.

But the government hasn’t completed updated modelling despite more than a month passing since scientific evidence recommended the longer gap.

Prof Murphy downplayed the significance of people having a single dose but is hopeful the two-shot target may still be achievable.

“It’s a semantic difference whether a small proportion have to come back a little bit later for a second dose. They will have been protected by the first dose,” he said.

Officials blamed the lack of updated modelling on the situation being dynamic with the impact from international supply and logistics.

Prime Minister Scott Morrison’s target of administering four million vaccines by the end of March appears to be dashed.

Murphy said it was “impossible to predict” when that mark could be reached with production in Melbourne and international supplies crucial.

Chief Health Officer Paul Kelly told the hearing just three of the 125,000 people who have received a jab displayed a severe allergic reaction.

All were dealt with soon after vaccination and suffered no long-term issues.

Morrison is sticking to the October goal despite experts setting the end of the year as a more realistic expectation.

“We will get this done by October as we said we would,” he told Seven.

“But we’ve got to do it safe – as Brendan Murphy said, this is not a race – what is most important is the health and safety of Australians in the vaccination program and that’s what we’ve principally focused on.”

Australia has now gone almost two weeks without a case of community transmission.

Murphy said the strong infection control performance meant the country was not rushing to administer vaccines.

Between the Pfizer and AstraZeneca jabs, there are 1.3 million doses of overseas-produced vaccines now in Australia.