This past week, COVID-19 returned to New Zealand. Just when Kiwis thought it was safe to go wherever they wanted (on their islands), more than a dozen people became infected.

But, there are signs Victoria may be over the worst.

The state's not out of the woods yet but the numbers are going in the right direction.

And in what may or may not be a further good sign, Russia announced its scientists had developed a vaccine.

So where do we stand?

We all thought New Zealand had cracked the coronavirus code.

There had been no new cases there for more than 100 days - so how on earth did the coronavirus sneak back in?

There are all kinds of weird and wonderful theories in the crazed world of social media, including one that it's all been set up by the New Zealand Defence Force so it can seize control of the country, with martial law imposed on October 1.

Michael Baker, professor of public health at the University of Otago, said the truth was likely to be much more obvious and simple.

He thinks the most probable source of the new bout was someone coming back from abroad, as people still have been in New Zealand, and then some sort of mistake within the quarantine system, perhaps in one of the 30 hotels turned into quarantine centres or at the airport among baggage handlers or other staff.

He likened it to Victoria, though he didn't blame anyone.

Mistakes get made and sometimes "a series of low-probability mistakes leads to a high-consequence event".

Once the virus was out in the community via the mistake, the family at the centre of the current outbreak contracted it, even though they hadn't been abroad.

Professor Baker discounted suggestions the virus came in on freight.

He thought it was more likely it came from a port to a cold-store depot where one of the infected people worked - but not carried on goods, rather as human-to-human contact.

But he still favoured the gap in the quarantine system theory.

The virus is still out there so there may be more community infection. A Victorian situation, where the numbers rise quickly, can't be ruled out.

The country has gone back into lockdown, with the toughest level in Auckland. Unlike the earlier outbreak, masks are now recommended.

There have been more than 26,000 tests done in the last two days.

Anyone who had come into contact with the infected family was being traced and tested.

People throughout the arrivals and quarantine system were also being tested.

And, finally, there has been widespread testing within the communities around the family, including in places they had been through.

Professor Baker's conclusion was: "You have to plan for resurgences. The key thing is to get them under control quickly."

After months of rising COVID-19 cases in Victoria, experts were expressing cautious optimism the state had turned a corner in its fight against the virus.

On Friday, there were 372 new cases in Victoria and on Thursday 278, the lowest daily total since July 20.

At its peak, there were more than 700 new cases a day, so it's progress and a tentative belief Victoria it may have "flattened the curve", at least until the next wave.

"There is a definite downward trend," epidemiologist professor Adrian Esterman of the University of South Australia said.

"It seesaws a bit on its way down, but it also did that on its way up.

"Importantly, I estimate the number will be close to 200 cases per day within a week."

But the crunchers of the numbers remain cautious.

"There's plenty of potential for the curve to turn back up again if adherence to the restrictions wavers," Ian Marschner, professor of biostatistics at the University of Sydney, said.

"Victorians have some reason to be optimistic that the peak has passed but there's no room for complacency."

Not so fast.

Russia said it had approved the world's first COVID-19 vaccine, dubbed "Sputnik V" after the first Russian satellite to be launched successfully into space.

President Putin, announcing the stunning publicity coup, said the vaccine was safe and his own daughter had been given it.

The Russian Ministry of Health had issued a "registration certificate" for a potential vaccine developed by a research institute in Moscow.

But the caveats take the shine off. It's only been tested on a small number of people and it's not clear how safe or effective it is.

Professor Adrian Esterman of the University of South Australia said the potential vaccine hadn't yet gone through phase three trials (on large numbers of people and thought to be essential before approval).

"There are already reports of adverse reactions in the small number of people that have taken part in the initial trials," he said.

The potential vaccine may well have indicated it could counter the virus but more needed to be done.

"Just because a vaccine produces an immune response does not mean it will necessarily be successful. In some cases, the immune response can itself be very dangerous," he said.

"This is why we need large phase three trials before we undertake mass vaccination."

The World Health Organisation said it was talking to the Russian authorities about reviewing the vaccine, which was not on its list of six vaccines to reach phase three trials.

Russia dismissed the doubts as sour grapes by the losers in the great global vaccine race.

"It seems our foreign colleagues are sensing the specific competitive advantages of the Russian drug and are trying to express opinions that are absolutely groundless," Russia's Health Minister Mikhail Murashko said.