Thousands of new cases of Covid-19 are being reported every day in New Zealand, but this is likely to be half of the number of actual cases, Dr Ashley Bloomfield says.
The Director-General of Health is giving an update on the response to Covid-19 and the Omicron variant.
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There were 7441 community cases reported today, but Dr Ashley Bloomfield said many cases are not being reported.
He said the Ministry of Health tried to estimate the level of reporting by looking at the positivity rate of the border workers who were undergoing constant surveillance testing.
"Based on that, we think that probably about half of actual cases are being reported. And some of those cases not being reported are because people don't have symptoms ... others the symptoms may be very mild so they don't get tested, whilst others may be testing, but not reporting the result."
With a further 29 deaths with Covid-19 today, Dr Bloomfield said the impact of the severity of Omicron was still visible.
"I mentioned a couple of weeks ago we're also looking at how those deaths are categorised and we're very nearly through that work."
He said death information would be updated daily to reflect the different categories of deaths - those definitely from Covid-19, those where Covid-19 was likely a contributing factor, and those where Covid-19 was likely incidental.
Cases per thousand have dropped from the peak for all regions, but have picked up a bit again in Northland and Auckland.
Dr Bloomfield said some of the increase may just reflect an increase in reporting, perhaps from people who went away over Easter.
"We have got increases in the number of cases in the seven-day period to 8 May. It increased in seven of our DHB regions."
Hospitalisations peaked about where modelling suggested, he said - about 1000 on 22 March - but the decline in hospitalisations had been lower.
"We thought our case numbers would come back to a baseline of around 3000 to 5000 a day, in fact they seem to have bottomed out at around 7500 a day. And hospitalisations have levelled off a the mid-300s.
"It seems we've probably reached the nadir, the bottoming out, of both cases and hospitalisations from this outbreak and we may well be on our way back up again."
In North America, cases have been rising again - starting on the east coast - and the variant driving most of that increase is the BA.2.12.1 subvariant.
"We have detected a few cases of that here in New Zealand, all having come from the USA, it now makes up 40 percent of cases in the USA and their hospitalisations are also increasing again."
In South Africa, the BA.4 and BA.5 subvariants have driven quite a large increase in cases, he said, and some of each of those have been seen in New Zealand.
The UK and Australia have experienced quite distinct Omicron outbreaks of BA.1 and BA.2, but New South Wales is following a similar pattern to New Zealand which have both experienced mostly BA.2 variants.
He said there was more emerging evidence about the effect of vaccination on people who had also been infected.
"Studies that have been published recently show that for people who have had Omicron infection, if they go and have a booster dose after a delay that affords a much higher level of protection than just the protection afforded by having been infected," Bloomfield said.
He said the advice was to wait three months before getting a booster, but it "is absolutely worthwhile ... to help protect yourself from potentially being reinfected and of course to protect others".
There was a small risk of being reinfected with Covid within 90 days, he said, but it was a small risk and the Ministry was continuing to monitor the evidence.
The government has been preparing for the onset of winter, which is expected to bring the usual winter illnesses that would add to pressures on the health system from Covid-19 and could increase severity due to comorbidity.
Modeller Michael Plank was this morning warning of the risks of a second wave as people's immunity from the virus wanes.
Epidemiologist Michael Baker said New Zealand had passed its peak level of immunity.
"That was shortly after most people had their boosters in particular, and also we'd rolled out the vaccine to children."
He said now the only ways people were able to replenish their antibodies was getting infected, or from booster doses - and the plans for a fourth shot would help.
Dr Bloomfield said the Ministry was preparing for another potential peak which could be quite high over winter, and was anticipating and planning for concurrent influenza and other infections.
Some cases of influenza have been detected in New Zealand including in the student population in Dunedin and in Queenstown. While flu cases in the northern hemisphere were quite low over winter, they have been increasing again now and Australia has also been seeing some cases.
"Flu and Covid at the same time, about around 3 percent of people they have found this in the UK and people can be particularly unwell with it."
He said it helped to get the flu vaccine and to be up to date with the Covid-19 vaccinations.
There was also modelling of RSV, respiratory illnesses and Covid all-19 at the same time, and a peak in the middle of winter was expected.
Dr Bloomfield urged people to seek flu vaccination if they are eligible or offered it, or seek it out even if they are not.
"It's very, very cheap."
Paediatrician Jin Russell told RNZ it was important for parents to think about vaccinating their children: against Covid-19 - including if they already had the virus - flu, and measles.
"A small proportion of children can get seriously unwell and it's those children we're trying to protect through vaccination," she said.
"We just don't know which variants are going to come next, this winter it might be a particularly difficult winter ... it's really important we try and take as much pressure off the healthcare system and try and keep as many people well from Covid as possible - and that includes children."
Dr Russell said all those vaccinations were particularly important for any children who had chronic conditions.
The government was also finalising eligibility and timing for a fourth dose. Dr Bloomfield said the technical advice was to wait until six months after the third dose, and - particularly important for older people - it was good to get people's resistance highest right during the winter-time peak of July and August.
He said the Ministry has been working hard - including with advertising - to try to get booster rates up, which "has been vexing us for quite a while".
The majority of cases were still in younger age groups, but the fastest increase in rate is in older age groups which explains partly why we are starting to see hospitalisations go up again.
Dr Baker said almost the most useful thing people could do was stay home if they were sick.
"This is still almost the universal vaccine for infectious diseases - it's not technically a vaccine but it is the universal protection - is stay put. Because these infect dis are extremely well adapted for transmission between people, otherwise they wouldn't be with us.
"The other universal precaution ... be very conscious about the fact that they're transmitted as aerosols indoors and take precautions and I think people know what they are. It's' making sure that you've got ventilation or if you are actually in a confined place that people are wearing masks. These precautions really do work."
The United States today is also marking more than a million deaths from Covid-19, the highest official toll in the world - although other countries may have higher tolls than are officially reported, according to the World Health Organisation.