Hilary Duff has been diagnosed with COVID-19.

The How I Met Your Father star, 33, shared the news Friday that she'd contracted a breakthrough case of the virus just days after production began for the upcoming How I Met Your Mother sequel series.

"That delta...she's a little bitch," Duff wrote on her Instagram Story, adding that she had a "bad headache" and had lost her sense of taste and smell. She also said she felt sinus pressure and "brain fog."

"Happy to be vaxxed," the actress concluded, adding a peace sign emoji.

Breakthrough cases — COVID-19 infections that occur in people who have been fully vaccinated against the virus — are rare, but possible and expected, as the vaccines are not 100% effective in preventing infections. Still, vaccinated people who test positive will likely be asymptomatic or experience a far milder illness than if they were not vaccinated. The majority of deaths from COVID-19 — around 98 to 99% — are in unvaccinated people.

Duff isn't the first celebrity to share that they had contracted a breakthrough case of COVID-19. On Thursday, Melissa Joan Hart revealed that she also had coronavirus after being fully vaccinated.

"I am vaccinated and I got COVID, and it's bad," she said told fans. "It's weighing on my chest. It's hard to breathe. One of my kids, I think, has it so far. I'm praying that the other ones are okay."

The actress — who shares sons Tucker McFadden, 8, Braydon Hart, 13, and Mason Walter, 15, with husband Mark Wilkerson â€” went on to explain that she and her family had taken precautions throughout the pandemic, but "got a little lazy" as restrictions were lifted.

"I think as a country we got a little lazy and I'm really mad that my kids didn't have to wear a mask at school. I'm pretty sure where this came from," she said. "I'm just scared and sad, and disappointed in myself and some of our leaders. I just wish I'd done better, so I'm asking you guys to do better. Protect your families. Protect your kids."

"It's not over yet," Hart added of the ongoing pandemic. "I hoped it was, but it's not, so stay vigilant and stay safe."