Breakthrough COVID-19 cases resulting in infections, hospitalisations and deaths are significantly more likely in cancer and Alzheimer's patients, according to two new studies.

"Breakthrough infection" is when a fully vaccinated person contracts COVID. Researchers at the Case Western Reserve University School of Medicine said that people with cancer and Alzheimer's are often more susceptible to infection in general, and are also among the population's most vulnerable to severe health outcomes from COVID infections as well.

The first study, published recently in the Journal of the American Medical Association (JAMA) Oncology, analysed electronic health records of more than 636,000 vaccinated patients, including more than 45,000 vaccinated people with cancer.

The team found the overall risk of breakthrough COVID infections in vaccinated people with cancer was 13.6 per cent, compared to 4.9 per cent for vaccinated people without cancer.

The highest risk of breakthrough infections was in people with pancreatic cancer at 24.7 per cent, followed by liver cancer (22.8 per cent), lung cancer (20.4 per cent) and colorectal cancer (17.5 per cent). Cancers with a lower risk of breakthrough infections included thyroid (10.3 per cent), endometrial (11.9 per cent), and breast (11.9 per cent).

The overall risk for hospitalisation following a breakthrough infection, in participants with cancer, was 31.6 per cent, while the risk of death was 6.7 per cent following a breakthrough infection.

"This study showed significantly increased risks for COVID-19 breakthrough infection in vaccinated patients with cancer, especially those undergoing active cancer care, with marked variations among specific cancer types," said Rong Xu, Professor of biomedical informatics at the varsity's School of Medicine.

In the second study, published in the journal Alzheimer's & Dementia, the team examined anonymous electronic health data from more than 262,847 adults 65 or older vaccinated between December 2020 and August 2021, and who didn't have the infection before being vaccinated.

Vaccinated patients with dementia showed an overall risk for breakthrough infections ranging from 10.3 per cent for Alzheimer's disease to 14.3 per cent for Lewy body dementia, significantly higher than the 5.6 per cent in the vaccinated older adults without dementia.

"Continued vigilance is needed, even after vaccination, to protect this vulnerable population. Caregivers should consider ongoing masking and social distancing, as well as booster vaccines to protect these individuals," said Pamela Davis, Professor at the School of Medicine.