Parents whose baby died from whooping cough suffer guilt over the arrival of their new daughter.

Riley Hughes was just 32 days old when whooping cough fatally ravaged his tiny lungs last March. It was a devastating tragedy that changed the lives of his parents, Greg and Catherine, but could end up saving those of other children.

The couple have worked tirelessly in the wake of the little boy’s death to ensure that people don’t succumb to vaccine-preventable diseases via government lobbying and the establishment of a new research foundation. And the birth of their new daughter, Lucy, is spurning them on.

On Monday the “energetic” little girl reached the 32-day mark, a day that was bittersweet for the Perth family.

“I think I spent most of that day cuddling her and watching her, just thinking about what awful things Riley was experiencing at that age,” Catherine told Kidspot.

“I now think of her as Riley’s ‘little big’ sister – she’s technically his little sister but is now older than him.”

Lucy Hughes. Via Light for Riley.

While Catherine says she coped well during Lucy's birth thanks to the support of hospital staff, a whirlpool of emotions swallowed her as she walked out the door.

“The last time we’d left a hospital, we were leaving a baby behind," she said. "Sometimes I stare at her and feel guilty that she’s alive and Riley isn’t. I know these aren’t very rational thoughts or feelings, but I just feel so bad that he’s gone.”

Their eldest, Olivia, who turns 5 in November, also feels and understands little Riley's loss more deeply than her years might suggest.

“She says Lucy is ‘her’ baby and loves to cuddle her and sing to her," said Catherine. "But we’ve had a few tough questions, like, 'Is Lucy going to die like Riley? Can we keep her forever?', and it shows she really knows what happened. It’s difficult.”

Babies can not receive their first whooping cough vaccination until they are six-weeks-old, but Catherine says that if a mother is vaccinated during pregnancy, her child is up to 90 per cent less likely to catch the disease.

Unfortunately, that wasn't offered to her.

But as a result of their tireless efforts with the Light for Riley campaign, the West Australian Government made whooping cough vaccines available free to every pregnant woman and by the end of last year every other state and territory had followed suit.

Riley Hughes. Via Light for Riley.

The couple's next focus is the Immunisation Foundation of Australia, an organisation they've launched to raise funds for awareness into vaccinations and research into preventable diseases.

“We envision a world where children don’t die from vaccine-preventable diseases, and we want to raise money for awareness campaigns and immunisation research," Catherine told Kidspot.

"We just want to do whatever we can to protect babies and children from diseases that can be easily prevented by a simple needle.”