Most recently, in the journal Nature, a study by scientists at University College London (UK) was published explaining the cause why are some people not infected with the SARS-CoV-2 virus despite exposure to F0.

Research results have shown that the reason why some people do not positive despite close exposure to F0 may be due to the response of T cells - white blood cells that play an important role in the immune system of humans’s body.

Many previous studies have shown that T cells fight off the SARS-CoV-2 virus by being able to recognize them from previous cold-like corona virus infections. However, it was not until this scientific work that showed that T cells affect the ability to become infected when a person is exposed to the SARS-CoV-2 virus.

"Exposure to the SARS-CoV-2 virus does not always lead to infection. We found that high concentrations of pre-existing T cells are produced by the body during infection with other human corona viruses may protect against COVID-19 infection." Rhia Kundu, Royal National Heart & Lung Institute (lead author of the study)

Scientists conducted a study on 52 volunteers exposed to SARS-CoV-2. (someone in their home has Covid-19) and re-tested at 4 and 7 days later, to determine if they are infected (confirmed by PCR). The result is negative. Study participants may also have been exposed to corona viruses such as the common cold virus in the past.

Explaining this, Professor Ajit Lalvani, a member of the research team shared: "Our study provides the clearest evidence to date that T cells fight the corona virus that causes colds. Normally play a protective role against SARS-CoV-2 infection". When the SARS-CoV-2 virus enters the body, in some people it activates T cells that remember previous illnesses, thereby responding to the virus. T cells target proteins inside the SARS-CoV-2 virus instead of spikes on their surface to fight infection.

Whether a person becomes infected after being exposed to Covid-19 depends a lot on the viral load to which the person is exposed, environmental conditions, the body's natural immune system, and the concentration of antibodies maybe present in the blood (through vaccination). 

Therefore, to ensure the safety of themselves, their families and society, everyone should still limit contact with others while they are infected, wear a mask in public and get tested as soon as they have been exposed or may be infected, contact with a person with Covid-19 or have symptoms are suspected.

Source: Ministry of Health website