At least five people have died in the United States after vaping, officials said on Friday, in an outbreak that has sickened hundreds with severe pulmonary disease and left several teens in induced comas.

Federal officials said that no single substance has been found to be present in all the laboratory samples being examined, however investigators in New York said they were now focusing on black market cannabis e-cigarette products containing vitamin E oil.

On Friday, local health authorities in California and Minnesota announced the vaping-related deaths of two individuals, both older and in relatively poor health, at least one of whom had used products containing THC, the principal psychoactive compound in cannabis.

There are now more than 450 possible cases of pulmonary illness associated with vaping, more than double the figure reported last week, according to Ileana Arias, acting deputy director for non-infectious diseases at the Centres for Disease Control and Prevention (CDC).

Daniel Fox, a pulmonologist in North Carolina, said that patients he had examined had a non-infectious pneumonia known as lipoid pneumonia, which can occur "when either oils or lipid-containing substances enter the lungs."

New York's health department said laboratory test results showed very high levels of vitamin E oil in cannabis cartridges used by all 34 people in the state who had fallen ill after using e-cigarettes, and as a result was focusing its investigation in that direction.

Vitamin E acetate is a commonly available nutritional supplement taken orally or applied to the skin, but is harmful when inhaled.

The Food and Drug Administration's acting administrator Ned Sharpless said his agency was aware of the reports, "but no one substance, including Vitamin E acetate, has been identified in all of the samples tested" at a nationwide level.

Many patients reported vaping cannabis, but some said they had only inhaled nicotine products.

'Don't do this'

The first death was reported in Illinois in late August. Oregon announced this week that the death of a patient in July was also being linked to vaping. And authorities in Indiana have said a death occurred there as well, although they did not mention when.

Patients have reported symptoms including breathing difficulty and chest pain before they were hospitalised and placed on ventilators.

Medics said many people had been initially misdiagnosed as having bronchitis or a viral illness until their symptoms escalated, at times to very extreme levels.

The parents of Kevin Boclair, a 19-year-old from Philadelphia, told local news CBS 3 Philly their son had been placed in a medically induced coma three weeks ago and may require a lung transplant if he recovers.

"I even know, as a nurse, he could die," said his mother Deborah Boclair. "So we are hoping it gets better, and I just want his friends to know and all these kids out there -- I could tell the parents, 'tell your kids don't do this.'"

Sean Callahan, the doctor who successfully treated 20-year-old Alexander Mitchell from Provo in Utah, told AFP: "I've never seen anything like this before.

"You know, when I've had people this sick who need this sort of life support, it's a really advanced case of like a flu, pneumonia or patients who have weakened immune systems from cancer and chemotherapy."

Many of those who have recovered received steroid treatments, though doctors are not sure whether it was the drugs that cured them.

Investigators are also uncertain about whether the cases have only been occurring recently, or were happening earlier but were not being linked to vaping because of a lack of awareness.

E-cigarettes have been available in the US since 2006 and are sometimes used as an aid to quit smoking traditional tobacco products like cigarettes.

Their use among adolescents has skyrocketed in recent years: Some 3.6 million middle and high school students used vaping products in 2018, an increase of 1.5 million on the year before.

For now, the outbreak appears to be confined to the United States, despite vaping's growing global popularity.