In the global outbreak's first wave, many critically ill patients in both countries were obese, although their death rates weren't higher than others. Many in both countries also were younger than those typically hard hit by seasonal flu, as has been found in the United States.
Patients studied worsened quickly after being admitted to hospitals. Most survived after intensive, lengthy treatment, although the death rate in Mexican patients studied - 41 percent - was much higher.
The reports were published online Monday in the Journal of the American Medical Association.
They aren't a true snapshot on prevalence. But a JAMA editorial says they provide clues on what hospitals elsewhere may see in coming months.
A report on U.S. cases published last week in the New England Journal of Medicine provided similar guidance. It found that one-quarter of Americans sick enough to be hospitalized with H1N1 flu (also known as swine flu) last spring needed intensive care and 7 percent died.
In the Mexican report on six hospitals between March and June, critical illness developed quickly in 58 of almost 900 patients with confirmed or suspected H1N1 flu patients - a rate of just under 7 percent. But 24 of these sickest patients died within two months, said the study led by Dr. Guillermo Dominguez-Cherit of the National Institute of Medical Sciences and Nutrition Salvador Zubiran in Mexico City.
The Canadian study, led by Dr. Anand Kumar of the Health Sciences Center and St. Boniface Hospital in Winnipeg, involved 168 critically ill patients treated at 38 hospitals between April and August. The 90-day death rate was 17 percent.
The JAMA editorial noted that while treatment including antibiotics, antiviral drugs and mechanical breathing machines has advanced since the deadly 1918-19 Spanish flu pandemic, many U.S. hospitals may lack adequate staffing levels to provide timely treatment if critical swine flu cases surge.
Deaths that result from inadequate planning "will be especially tragic," the editorial said.