27 July 2020
Among the many terms which have entered the lexicon of public discourse since the emergence of SARS-CoV-2, ‘asymptomatic carrier’ has perhaps been the most fear-provoking.
The notion that your co-worker, your neighbour, the cashier who served you at the supermarket, or even a member of your own family may be capable of infecting you with SARS-CoV-2 without showing the slightest symptom of illness themselves, has been enthusiastically disseminated by both the media and public health authorities. It has also driven public policy decisions such as the mandating of mask-wearing in public spaces, in many jurisdictions.
But what exactly is an ‘asymptomatic carrier’, and are they really a grave threat to public health – and your personal health?
A person who tests positive on a reverse transcriptase polymerase chain reaction (RT-PCR) test for SARS-CoV-2, but has no symptoms of COVID-19 (fever, dry cough, fatigue, loss of appetite, body aches, shortness of breath, loss of smell or taste, nausea, vomiting, diarrhoea) either at the time of testing, or after a follow-up period lasting at least 7 days (and ideally, longer), is considered to be an asymptomatic carrier.
If you’ve already read my previous article COVID-19: To test or not to test, this definition of an asymptomatic carrier should already be setting off alarm bells. As mentioned in that article, PCR testing was not developed to diagnose viral infections, and a positive PCR test does not indicate contagiousness. The PCR test simply identifies the presence of a specific fragment of viral genetic material.
People who develop the SARS-CoV-2-related illness, COVID-19, may continue to test positive for the presence of the virus weeks, and possibly even months, after making a full recovery; however, these recovered people are not contagious despite their positive RT-PCR tests.
Meanwhile, individuals can repeatedly return a positive result on the RT-PCR test for up to 2 weeks (and quite possibly longer), without developing any symptoms of illness.
How can this be? Remember, viruses are intracellular pathogens: they have to gain entry to our cells, and hijack our cellular machinery to make copies of themselves, in order to make us sick.
Our immune systems are able to detect the altered appearance of virus-infected cells, and in order to contain the infection, immune cells called natural killer cells and cytotoxic T cells kill the infected cells.
The killed cells rupture and spill their contents, including viral particles, into surrounding tissue. This results in the presence of virus fragments in respiratory secretions, which are the most common samples used for the RT-PCR test for SARS-CoV-2.
In other words, PCR tests detect fragments of viral genetic material (RNA, in the case of SARS-CoV-2) but they do not ascertain the presence of infectious virus.
In order to determine whether the viral fragments found in respiratory secretions are actually capable of causing an infection in someone else, the specimens need to undergo viral culturing, a process in which samples of respiratory secretions are inoculated into cells to see if the cells develop the hallmarks of viral infection.
Very few such culturing studies have been performed, but of those that have been done, there is scant evidence that asymptomatic people harbour virus that is capable of infecting others.
For example, two individuals who returned to Germany on a repatriation flight from Wuhan, China displayed no symptoms of infection either before the repatriation flight or during the 2 week quarantine period after returning to Germany, but showed positive results on the RT-PCR test administered after the flight. Viral culturing was carried out on the specimens that tested positive, and viable virus was cultured from both individuals.
However, despite being at close proximity to 114 other individuals during the repatriation flight, there was no evidence that these ‘asymptomatic carriers’ infected any of their fellow passengers, who were released from quarantine 2 weeks later with no signs or symptoms of COVID-19.
Of the reports which have relied on epidemiological evidence that asymptomatic carriers may be able to infect others, all are dogged by potential confounding from community transmission – that is, it cannot be proved that people who contracted SARS-CoV-2 after contact with an asymptomatic carrier developed infection from the asymptomatic carrier rather than other infected people.
The authors of a systematic review and meta-analysis of studies of asymptomatic carriers assessed epidemiological reports (none of which included viral culture to detect infectious virus) and found that the rate of transmission from asymptomatic carriers ranged from 0% to 2.2% – that is, at most, the risk of contracting SARS-CoV-2 infection from a so-called ‘asymptomatic carrier’ ranges from 0 to 1 in 50. They concluded that:
“Our estimates of the proportion of asymptomatic cases and their transmission rates suggest that asymptomatic spread is unlikely to be a major driver of clusters or community transmission of infection.”
Estimating the extent of asymptomatic COVID-19 and its potential for community transmission: systematic review and meta-analysis
In a study of 4950 close contacts of people who tested positive for SARS-CoV-2 in Guangzhou, China, the risk of developing COVID-19 ranged from 0.33% in those who were believed to acquire their infection from an asymptomatic carrier (that is, a 33 in 10 000 risk of becoming ill), to 3.3% in those whose contact had a mild case of COVID-19, to 6.2% for those whose contact had a severe or critical case.
During their 2-week quarantine period, 129 cases were diagnosed from among the 4950 close contacts (that is, 2.6% of the contacts tested positive for SARS-CoV-2); 8 (6.2%) were asymptomatic, 49 (38.0%) were mild, and 5 (3.9%) were severe to critical cases.
The case of a 22 year old Chinese woman who tested positive to SARS-CoV-2 on admission to hospital for treatment of her congenital heart condition, but remained asymptomatic throughout her one-month stay, is particularly instructive.
455 contacts of the young woman, including family members, patients and hospital staff, were identified. The family members and hospital patients were quarantined and the staff remained under observation for 2 weeks. Not a single one of these 455 contacts returned a positive RT-PCR test for SARS-CoV-2 or developed any symptoms of COVID-19.
The bottom line: There is scant evidence that people who return a positive result for SARS-CoV-2 on the RT-PCR test but do not develop symptoms of illness are able to transmit the virus to other people, or make them unwell. While further studies that involve viral culturing of so-called asymptomatic carriers are needed in order to bring clarity to this contested issue, it is extremely unlikely that asymptomatic carriers are contributing significantly to viral spread. Media fear-mongering of the spectre of COVID-19 ‘Typhoid Marys’ has not only ramped up public anxiety, but has also significantly contributed to the formation of public policy (including mandating of mask-wearing) that is not solidly grounded in the scientific evidence.
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