There are 2 types of tests. One tests looks for the virus and it is called an antigen
test. The other tests looks for a history of exposure to the virus and it is an
antibody test. There are 2 types of antibodies and they mean different things.
Antigen Test looks for the Viral RNA. This is the test where the oropharynx is
sampled with a long nasal swab. This test is best for new onset of illness and
determines if a person is contagious. This test can also find asymptomatic carriers
of the virus. The yield on this tests decreases over time as the virus migrates away
from the oropharynx and towards the lungs.

Antibody Test looks for the bodies response to the virus. After 5 days of exposure
to the virus, the body begins making an antibody called IgM. This antibody stays in
the body for 4 weeks and if a person has just IgM and no symptoms, they are
assumed to be contagious for 7 days from the time of the test. The body also
makes another antibody called IgG and the body begins making IgG about 2 weeks
after being exposed to the virus and then stays in the body for months to years.
The presence of IgG is the bodies best hope for having long lasting immunity.
Therefore, a company who wants to improve the safety of their work environment
prior to opening would screen all of their employees looking for asymptomatic
carriers. You could either perform a nasal swab looking for the virus and/or an
antibody test looking for the presence of IgM antibody without the presence of IgG.
Any employee who is demonstrating any symptoms of an illness would of course
self quarantine and would be tested within the first couple of days with a nasal
swab looking for the viral RNA.

A physician is able to stratify the risk based on current or historical symptoms and
exposure history. A physician is also able to help interpret the tests in order to
manage the work environment. COVID is a unique virus in that it has the ability to
infect individuals through aerosolized droplets, contact and oral-fecal transmission.
A work place is able to mitigate these risks by maintaining 6 feet of space, wearing
masks and good hand hygiene. COVID transmission benefits from time and proximity.
The longer people spend together the more likely they art to infect one another.