Jasper Allen spent five days in hospital after severe chickenpox sores - which covered his entire body - became severely infected.
His mum Sarah, 36 and from St Neots, Cambridgeshire, wants the vaccination made available free on the NHS.
The Department of Health said its vaccination policy would be reviewed.
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The jab is currently only available to certain children on medical grounds.
Nursery manager Mrs Allen said pictures of her son's condition surprised everyone who saw them.
"People say their child had bad chickenpox, but when we show people pictures of Jasper none of them can believe just how bad it was," she said.
"Everyone's reactions in the hospital were just complete shock over how severe it was - the doctors all wanted to come and see this worst ever case of chickenpox."
Mrs Allen and husband Keith, who also have five-year-old daughter, first noticed a few spots on Jasper after he had recovered from scarlet fever the week before.
But, by the next morning he had developed "hundreds of spots" and had a high temperature.
As Jasper's temperature continued to rise, Mrs Allen took him to their GP and he was prescribed antibiotics and oral medication.
But, his condition did not improve, prompting his parents to take him to A&E at Hinchingbrooke Hospital in Huntingdon. He spent five days in hospital on an IV drip and antiviral medication, antibiotics and morphine.
Mrs Allen said doctors did not know why Jasper's chickenpox was so severe but he is currently undergoing heart scans to make sure there are no lasting effects.
Meanwhile, more than 2,500 people have signed Mrs Allen's petition to make the chickenpox vaccination part of the NHS's routine childhood immunisation schedule.
Source: NHS Choices
A mother has called on the government to make the chickenpox vaccination free for all after her toddler had what she claims was the "worst ever case".