Across the globe, official public health policy during the COVID-19 pandemic has been underpinned by the concern that people without disease symptoms may transmit the virus. This has led to recommendations such as universal mask-wearing, social distancing, mass testing, stay at home orders, and school and business closures.
“Searching for people who are asymptomatic yet infectious is like searching for needles that appear and reappear transiently in haystacks, particularly when rates are falling.” (Ref)
The concern that SARS-CoV-2 could be spread by people without symptoms originally came from a single case report. It was alleged that an asymptomatic woman from China had spread the virus to 16 other contacts in Germany. Later reports showed that, at the time of contact, this woman was actually taking medication for flu-like symptoms, invalidating the evidence provided for the theory of asymptomatic transmission.
As with other common respiratory viruses, SARS-CoV-2 spreads by being exhaled, coughed or sneezed into the air (Ref). The largest droplets fall quickly and settle on the ground whilst the most lightweight particles, known as aerosols, may remain suspended in the air for days (Ref). Once the virus is present in the environment, it spreads by finding its way into the respiratory tract of new hosts in a large enough quantity (known as the ‘viral load’ or ‘infectious dose’) to infect them. The theory of fomite transmission (touching contaminated surfaces and then touching the face) is not supported by scientific evidence (Ref).
The most significant risk factor for COVID-19 disease is advanced age and the presence of underlying health issues such as cardiovascular disease, obesity and type 2 diabetes (Ref & Ref). Both factors contribute to a frail immune system. In addition to the health status of the exposed person, the environment in which exposure occurs also affects the probability of that person falling ill. Infectious aerosols remain suspended for longer in cold, dry air. Hence respiratory viruses transmit most efficiently during colder seasons (Ref). People spend more time indoors during cold weather, where poor ventilation leads to higher concentrations of infectious aerosols remaining in the air. Spending time in crowded indoor spaces also increases the risk of transmission (Ref, Ref, & Ref). Furthermore, lack of exposure to the sun in colder weather results in lower Vitamin D levels, and greater susceptibility to illness if infected (Ref).
Infection with the SARS-CoV-2 virus causes some individuals to become ill with COVID-19. Many people have had previous exposure to other related coronaviruses. These individuals develop mild or no symptoms following infection with SARS-CoV2, most likely due to protection conferred by this exposure. Cross-immunity has been demonstrated in multiple studies (Ref, Ref, Ref, & Ref). “The evidence that a subset of people has a cross-reactive T cell repertoire through exposure to related coronaviruses is strong.” (Ref).
People presenting with symptoms of COVID-19 are almost exclusively responsible for transmitting SARS-CoV-2. Serious infection usually results from frequent exposure to high doses of SARS-CoV-2, such as health care workers caring for sick COVID-19 patients in hospitals or nursing homes (Ref) and people living in the same household (Ref). ”The risk for transmission of SARS-CoV-2 among close contacts increased with the severity of index [initial] cases.” (Ref). The more serious the individual’s symptoms are the more infectious they are.
A person showing no symptoms of COVID-19 may test positive for SARS-CoV-2 on a PCR test, which doesn’t necessarily mean that they are infectious. There are four ways in which this can happen:
The test may give a false positive result due to several faults in the testing process or in the test itself (Ref) (the person is not infected);
The person may have recovered from COVID-19 in the last three months (the person is not currently infected but dead debris of the virus are being picked up by the test);
The person may be pre-symptomatic, i.e, the person is infected but still in the early stages of the disease and has not yet developed symptoms; or
The person may be asymptomatic, i.e. the person is infected but has pre-existing immunity (Ref) and will never develop symptoms.
In asymptomatic individuals, the viral load is typically very low and the infectious period is also short in duration. They may still exhale virus particles, which another person may encounter. However, the overall likelihood of transmitting the disease to others is negligible. Thus asymptomatic cases are not the major drivers of epidemics.