In this project, adults living in Norway will be invited to a study to investigate whether cod liver oil prevents Covid-19 and complications of Covid-19. Preliminary univariate and partially multivariate results from Koronastudien.no, which is a case-control and cross-sectional study performed by our research group, shows that participants who used cod liver oil had a lower incidence of Covid-19 and that those who received Covid-19 had a milder disease, and they were less frequently hospitalized. Data from other research also suggest that vitamin D found in cod liver oil may have such an effect. It has been previously shown that vitamin D can prevent respiratory infections and that people with severe Covid-19 often have low levels of vitamin D. Nevertheless, it is far from clear that Covid-19 can be prevented with cod liver oil (or vitamin D) and this is what we will try to show in a study with good study quality. The study will be carried out such as neither the participants nor us conducting the study know whether each participant receives cod liver oil or placebo (corn oil) with added taste. Afterwards, we will see how many people in each group received Covid-19 and how it went with the participants who received Covid-19. To get the answers reasonably quickly, many participants must participate in the study. This will be a large unique study where both advantages and disadvantages of cod liver oil can be examined in great detail. We will therefore also look at other health effects, such as colds and more serious diseases both in short and long term in the two groups. More than 5 million bottles of cod liver oil are sold each year and the study is suitable to learn more about cod liver oil because the participants are randomized to either cod liver oil or placebo.We will collect information about those who participate in the study from the participant himself and from several Norwegian registers that have relevant information, such as the patient register, which has information about hospital admissions and the reason for these. Some participants will also be invited to provide a blood sample that will be used, among other things, to measure Covid-19 antibodies and levels of vitamin D and omega 3. The project will make use of surplus material from other blood samples as much as possible to reduce strain on participants and resource use. We believe that the project will show whether cod liver oil can prevent Covid-19 and serious illness among those who receive Covid-19. This would be very useful information, because there is currently no particularly effective treatment or vaccine against this disease. The project is also calculated to be able to say whether cod liver oil prevents other respiratory infections.