Taking supplements to avoid or reduce the severity of COVID-19 is controversial. However, vaccines are never 100% effective and one does not exist against SARS-CoV-2 (the virus). Please view these sites for information on the flu vaccine and its surprising potential impact on SARS-CoV-2.
Dr. John Campbell of the U.K. has produced a compelling video on the value of vitamin D against respiratory infection that provides up to 70% protection against respiratory infections caused by either bacteria or viruses. https://youtu.be/W5yVGmfivAk
The Linus Pauling Institute that spun out from the work of 2-time Nobel Laureate, Dr. Linus Pauling offers compelling information on the anti-viral activity of vitamin C.
There is no substantial evidence on the action of vitamin C against SARS-CoV-2, but the Pauling Institute indicate that appropriate daily doses of vitamin C reduce the severity of the common cold which may be caused by types of coronaviruses.
In general, supplements provide micronutrients at higher levels than found in most foods. Many of these nutrients are shown to boost immunity.