Thousands of people will be cancelling their Christmas Day plans thanks to the invisible grinch, Covid-19.

Leading epidemiologist Michael Baker estimates 85,000 people will be in isolation in three days' time.

He says gathering outdoors or in well-ventilated spaces is key to limiting the Christmas spread of Covid - and testing beforehand.

"No-one will thank you for turning up and infecting other people, particularly if there are vulnerable people there. This is a time to be responsible and test if you have got symptoms, and then act accordingly."

Crunching the numbers, Baker says we could expect about 12,000 new infections on Christmas Day, based on the daily average of reported cases, plus the same number again of unreported ones.

Covid Modelling Aotearoa programme co-leader Dion O'Neale agrees.

"We're sitting at the peak of a relatively decent-sized wave at the moment, so definitely lots of people will end up missing Christmas because they're a confirmed case and will have to isolate."

He expects reported case numbers to decrease, but reminds people not to rely on that as a signal the wave is over.

"They just don't report a case when they're having a fun time, that's almost certainly happened this week with schools knocking off and a bunch of people leaving work."

'We have had to actually cancel Christmas'

One Auckland man, who wished to remain anonymous, says Covid has slipped through the chimney at his house - he has two family members who tested positive this week.

"Sadly we have had to actually cancel Christmas. We had been really looking forward to getting together with my sister and her kids for a big family get-together… and I had to phone her yesterday and say, 'Look, I'm really sorry we can't do it, it's all off'."

They will take Christmas Day as it comes and delay their family gathering.

"We're just going to have to try and make it as nice as we possibly can, depending how people are feeling. It could be that some people are feeling unwell."

Auckland woman Melanie Bruges will get out of isolation in time to celebrate Christmas Day with family.

"We're having family over on Christmas Day on Sunday, so I'm going to keep a really low-profile until then. We'll probably test on Christmas Day before everybody comes over."

If her husband or their seven-year-old tested positive, they will postpone.

"We've got five grandparents around for Christmas Day and we wouldn't want them to be exposed to anything just for the sake of a meal. We can always put it off."

Free biscuit not worth the risk

For the thousands who were flying to their Christmas Day destination, O'Neale says it pays to be cautious and mask-up.

"Is it really diminishing your travel experience if you don't get your free glass of water and a dry biscuit on the plane? Would you rather have a dry biscuit or Covid?"

Michael Baker. Photo: RNZ.

He and Baker don't want the grinch to steal Christmas.

"It's absolutely essential for your health, wellbeing and enjoyment of life to get out and reconnect with your family and friends and have an enjoyable summer, that is so important," says Baker.

"Covid should not get in your way at all, and it's a matter of making small changes in how you do things just to make it a lot safer for everyone."

-RNZ.