* history of renal failure or dialysis, hypercalcemia, severe liver disease (cirrhosis), sarcoidosis or other granulomatous diseases (wegener) * allergy to fish or corn oil. * pregnancy or planned pregnancy before summer 2021 * vegan diet * age \>75 years old at inclusion based on the norwegian personal identity number * difficulty in swallowing cod liver oil or other oils * previous covid-19 disease * for caucasians only: use of any supplement containing more than trace amounts of vitamin d or omega-3 fatty acids at inclusion (vitamin d levels in non-caucasians living in norway are frequently low even among those self-reporting using dietary supplements). this criterion will be relaxed if too few participants volunteer for the study and only caucasians that use cod liver oil or an equivalent dietary supplement (with omega 3 and vitamin d) more than 5-7 times per week will be excluded.

* history of renal failure or dialysis, hypercalcemia, severe liver disease (cirrhosis), sarcoidosis or other granulomatous diseases (wegener) * allergy to fish or corn oil. * pregnancy or planned pregnancy before summer 2021 * vegan diet * age \>75 years old at inclusion based on the norwegian personal identity number * difficulty in swallowing cod liver oil or other oils * previous covid-19 disease * for caucasians only: use of any supplement containing more than trace amounts of vitamin d or omega-3 fatty acids at inclusion (vitamin d levels in non-caucasians living in norway are frequently low even among those self-reporting using dietary supplements). this criterion will be relaxed if too few participants volunteer for the study and only caucasians that use cod liver oil or an equivalent dietary supplement (with omega 3 and vitamin d) more than 5-7 times per week will be excluded.

- history of renal failure or dialysis, hypercalcemia, severe liver disease (cirrhosis), sarcoidosis or other granulomatous diseases (wegener) - allergy to fish or corn oil. - pregnancy or planned pregnancy before summer 2021 - vegan diet - age >75 years old at inclusion based on the norwegian personal identity number - difficulty in swallowing cod liver oil or other oils - previous covid-19 disease - for caucasians only: use of any supplement containing more than trace amounts of vitamin d or omega-3 fatty acids at inclusion (vitamin d levels in non-caucasians living in norway are frequently low even among those self-reporting using dietary supplements). this criterion will be relaxed if too few participants volunteer for the study and only caucasians that use cod liver oil or an equivalent dietary supplement (with omega 3 and vitamin d) more than 5-7 times per week will be excluded.

- history of renal failure or dialysis, hypercalcemia, severe liver disease (cirrhosis), sarcoidosis or other granulomatous diseases (wegener) - allergy to fish or corn oil. - pregnancy or planned pregnancy before summer 2021 - vegan diet - age >75 years old at inclusion based on the norwegian personal identity number - difficulty in swallowing cod liver oil or other oils - previous covid-19 disease - for caucasians only: use of any supplement containing more than trace amounts of vitamin d or omega-3 fatty acids at inclusion (vitamin d levels in non-caucasians living in norway are frequently low even among those self-reporting using dietary supplements). this criterion will be relaxed if too few participants volunteer for the study and only caucasians that use cod liver oil or an equivalent dietary supplement (with omega 3 and vitamin d) more than 5-7 times per week will be excluded.