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Dr Amir Khan

Many of us will be familiar with this scenario: Someone we know has tested positive for Covid-19 but despite living with that person, one or more members of their household continue to test negative.

How is this possible? We know that SARS-CoV-2 is mainly spread through airborne particles. We have also seen how quickly it can spread through communities. So, how then can the people breathing the same air as those infected, remain uninfected?

According to a new study, the answer may lie in people being exposed to other coronaviruses in the past that enabled them to make memory immune cells, called T cells, which offer protection against the SARS-Cov-2 coronavirus.

The term coronavirus is a broad one that encompasses a number of viruses in addition to the one that causes COVID-19.

Many of the common colds we see are caused by viruses that belong to the coronavirus family.

The study suggests that memory T cells created by exposure to these viruses could be why some people test negative for COVID-19 despite living with someone who has tested positive.

Although the different coronaviruses cause different illnesses and the viruses behave in different ways, they do have some shared characteristics that group them together into the same family.

Their shared structural similarities enable immune cells that recognise one type of coronavirus to recognise another type.

The study followed 52 Covid-19 household contacts. Its aim was to capture the earliest point at which these people developed an immune response to the SARS-Cov-2 virus.

Blood was tested for immune cells between day one and day six of when their household contact tested positive for Covid-19.

They found higher levels of memory T cells in the samples of those who tested negative on a PCR test for Covid-19 than those who tested positive.

The authors believed the levels and speed at which the memory T cells became active after being exposed to their household contact of Covid-19 suggests these immune cells were pre-existing from previous coronavirus infections, and not from the current exposure to the SARS-Cov-2 virus.

This is why this group of people did not test positive on a PCR test. They believe these pre-existing T cells triggered an immune response that quickly dealt with the SAR-CoV-2 virus before it could infect the individual and show a positive PCR test. They did also find these T cells in the group that tested positive but the levels were not as high.

The T cells that they measured not only targeted the spike protein part of the virus but also the nucleocapsid, the core part of the virus that stores its genetic material.

They concluded that future vaccines against the SARS-CoV-2 virus may need to induce a wider immune response than many of the current ones, which create antibodies to the spike protein part of the virus only. Hence they are more likely to remain effective even if new variants arise with new mutations.

As a result, the next generation of Covid-19 vaccines may target core coronavirus proteins as well as the outer spike protein.- Aljazeera