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The 47-year-old journalist confirmed that his son has tested positive for the virus.
"A few days ago, I woke up but my body refused to move in any direction. I had a headache, my body was aching like something would break if I moved suddenly, and I had a frog in my throat," Garner wrote in a column for NBR.
"The next day I was close to being okay, so texted the ex-wife to organise picking up our son Buster for the afternoon. He's asleep, sick, she said. It then made sense," Garner wrote.
"Everything he was suffering from pointed towards Covid-19. So, we got him tested. Results confirmed Covid. My results are pending but, at the time of writing, all roads appear to lead to Covid-19. I can't taste anything, I'm sweating, I'm feeling hot".
Garner, who is double-vaccinated, said his 11-year-old son wasn't old enough to get the vaccine and urged for people to have the jab.
"Get vaccinated. You're a mug for not doing it," he wrote.
Since the Delta outbreak began in August this year, 938 children under the age of nine have caught the virus. Fifteen children under nine have been hospitalised. This represents 5% of all hospitalisations.
Of the 90 cases currently in hospital with Covid-19, 50 are unvaccinated or ineligible, nine have been partially vaccinated for less than 14 days and another 14 have been partially vaccinated for more than 14 days.
Garner has spent more than 20 years working across television and radio, most recently as the host of the AM Show.
MediaWorks confirmed earlier this month that Garner, along with Rachel Smalley, will be moving to its new talk radio brand.
The pair will join current Newshub political editor Tova O'Brien, who was also announced as the station's breakfast host earlier this month.
Garner said in August he had decided to leave the "best job ever" at the AM Show because he was struggling with the "brutal hours".
"I have a particularly heavy heart leaving our AM team and have loved being on air daily with Amanda and Mark. They are brilliant, fun co-hosts and I trust them immensely.
"This place has been home for 20 years and it's bloody hard to say goodbye but I have to. I have a few important family and personal things that need my attention and now is the right time to go."