Professor David Skegg believes New Zealand's Delta outbreak could mean a long lockdown - and says the fight against the virus will go on for years.
The epidemiologist said he had not been closely working on the latest cases but "as an uninformed person, it is unlikely the lockdown will end next Tuesday. It'll be a matter of weeks I fear".
Skegg told Parliament's health select committee that opening borders would mean more outbreaks of Covid-19.
"Let me be completely frank - when we reopen the border, things are going to get tough.
"When we start reopening the borders, we are going to have outbreaks of Covid-19 and they're going to be difficult to control.
"We are going to be at war with this virus for years," Skegg said.
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The health committee discussed New Zealand's reopening strategy and the latest outbreak.
"Our number one priority is to stamp out this outbreak that we are experiencing, and our other priority is to vaccinate as many people as possible," Skegg said.
Regarding Australian Prime Minister Scott Morrison's comments about New Zealand's elimination strategy, Skegg said he "thought the comments from Scott Morrison were pretty inappropriate".
National's Covid-19 spokesman Chris Bishop asked Skegg what sort of vaccination coverage was needed to reopen to the world.
"It didn't make a lot of sense to pluck a figure out of the air like the Australians have like 70 per cent or 80 per cent," Skegg said.
"The more people you have vaccinated the better - the more options you've got.
"With this pesky virus you don't reach a state of herd immunity," Skegg said.
Bishop asked whether Skegg whether he could say how vaccination rates would affect the severity of outbreaks.
"Unfortunately there is no simple answer," Skegg said.
"Even at over 90 per cent we couldn't just rely on vaccination. A lot depends on how many people we have coming in carrying the virus and whether we have to adopt physical measures like mask wearing, it depends how good out contract tracing is," Skegg said.
"I'd love to be able to answer your question, but there's no way it can be answered.
Act leader David Seymour asked the committee about booster shots.
Professor Nikki Turner, of the University of Auckland, said the evidence on when boosters would be needed was unclear. Evidence so far suggested immunocompromised people were likely to need boosters first.
"The early data coming through is that some immunocompromised people are likely to see waning immunity first.
"We are expecting at some stage we will need boosting, but we do not know when."
Skegg said the Delta variant makes him "less optimistic that next year we will be able to continue controlling outbreaks by testing and contact tracing alone".
This is because the period between infection and becoming symptomatic is so short, it is very difficult to contact trace.
Turner discussed why it was important the Government was now extending the time between first and second doses of the vaccine.
She said that one dose of the vaccine provided people with significant protection.
"In the end our priority is if we do get community transmission we want as many people as possible protected against severe disease," Turner said.
Turner said that the vaccine is still quite "difficult" for primary care providers to manage, because of its temperature requirements and other issues. This has made it more difficult to bring GPs and pharmacies online.
Skegg said the country will have to get more used to using the bluetooth function of tracing if we're to get on top of reopening.
"That's the kind of thing we're going to really lift our game on," Skegg said.
Turner said the Government could look at using other non-Pfizer vaccines in future if the evidence suggested they were worthwhile.
"We need to watch this space," Turner said.
Turner said that for the small number of people who had a bad reaction to the Pfizer vaccine, the Government would look to use other vaccines, likely the Jansen vaccine.
Bishop asked whether the country may have to ditch the elimination strategy because it became impossible to maintain.
"What would it take for something to happen in New Zealand for you to conclude that elimination is now an impossible goal?" Bishop said.
Skegg replied that the country will "continue to try and stamp it out. When we fail - we may fail, we've taken an ambitious strategy, but we lose nothing by doing that.
"We may find that it's not possible, we may find that we have to move to a suppression strategy, but it won't be something we do in advance - it will just happen.
Skegg said that life under a suppression strategy would be more difficult than life under elimination.
"Unfortunately, it will not be good because we will all have to live a more restrictive life, people will have to shield from each other particularly in winter," Skegg said.
The National Party today launched a five point vaccination strategy, which focused on targeting vaccinations at people who were most vulnerable, targeting vaccinations at the vectors of transmission - like young people, and ramping up the rate of vaccination to 100,000 a day.