Many of us will be familiar with this scenario during the pandemic. One member of the family starts to experience the telltale signs of coronavirus, gets tested, and returns a positive result.
Then, after a day or so, another person in the same household also gets struck down with the virus, followed by another. Yet despite breathing in the same air and using the same living room, kitchen, bathrooms and bedrooms, there is one lucky soul who manages to avoid catching it entirely.
But why does this happen? Richard Stanton, professor of virology at the School of Medicine at Cardiff University, said there are many potential reasons why someone might not transmit Covid-19 to a family member.
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1. The strain of Covid: "It can depend on the virus. Omicron is more transmissible than Delta and so we see that family members are more likely to be infected if someone in the household has Omicron compared to Delta."
2. Genetics: "People have differences in how good their immune system is due to having different genes. Some people's immune systems are just 'better' at fighting off the virus."
3. Previous infections: "It's possible that a previous infection with certain common cold viruses can leave you with enough immune 'memory' to handle Covid better."
4. Chance: "Finally, it might just be chance, based on how much virus the infected person has in their mouth, and how often (and how closely) others interact with them."
Despite Prof Stanton's anecdotal findings the true understanding is still being investigated by infectious disease specialists across the globe. An increasing amount of research is being devoted to the reasons why some people never seem to get Covid – a so-called 'never Covid' cohort.
In January new research was published by Imperial College London suggesting that people with higher levels of T cells – a type of cell in the immune system – from common cold coronaviruses were less likely to become infected with SARS-CoV-2, the virus that causes Covid-19.
Dr Rhia Kundu, first author of the study from Imperial's National Heart and Lung Institute, said that "being exposed to the SARS-CoV-2 virus doesn't always result in infection and we've been keen to understand why". She said: "We found that high levels of pre-existing T cells, created by the body when infected with other human coronaviruses like the common cold, can protect against Covid-19 infection."
However Dr Kundu also cautioned that while this is an important discovery it is only one form of protection and added: "I would stress that no-one should rely on this alone. Instead the best way to protect yourself against Covid-19 is to be fully vaccinated, including getting your booster dose."
Covid vaccines have been proven to reduce severe infections, hospitalisations, and deaths and remain largely effective against known variants of the virus. However they are not 100% effective in preventing infection and the immunity they provide wanes over time.
Andrew Freedman, an academic in infectious diseases at Cardiff University, told CNBC that why some people get Covid and others don't "is a well-recognised phenomenon and presumably relates to immunity from vaccination, previous infection, or both".
He added: "We know that many people have still caught (mostly mild) Omicron infection despite being full vaccinated, including [having had] a booster. However vaccination does still reduce the chance of catching Omicron and responses do vary from person to person. So some people catch it and others don't despite very significant exposure."
As mentioned by Prof Stanton the answer to a person not catching Covid might lie in our genes. Imperial College London has carried out research into immunogenetics (the relationship between genetics and the immune system) and Covid-19 infection. They have found that variations between people's immune systems "makes a difference at least to whether or not you get symptomatic disease".
The research is focused on different human leukocyte antigen (HLA) genes and is looking at how these can affect a person's response to Covid with some HLA types more or less likely to experience a symptomatic, or asymptomatic, infection. Danny Altmann, professor of immunology at Imperial, said: "The key genes that control your immune response are called HLA genes. They matter for determining your response on encounter with SARS-CoV-2. For example people with the gene HLA-DRB1*1302 are significantly more likely to have symptomatic infection."
There could also be an issue with the Covid tests themselves. PCR tests are best at detecting the virus within the first 10 days of developing symptoms with earlier testing increasing the likelihood of detection. PCR test accuracy is upwards of 90%. However the efficacy of Covid-19 tests is still being reviewed and can depend on factors such as whether the test was administered correctly and, in the case of PCR tests, how soon after the swab was taken the sample was analysed.
It also appears that in people with low viral loads – where the virus has not replicated as dramatically in their body so they have fewer virus particles – PCR tests can give a false negative result. People who are vaccinated are much more likely to produce low viral loads and milder symptoms if infected.
Director of public health pathology at New South Wales Health Pathology, Prof Dominic Dwyer, told The Guardian : "Sometimes a confirmed Covid case in a household prompts everyone to get tested and those other people may have cleared the virus from their system already even if they have some mild symptoms. So testing can be complicated."
