Coronavirus infection: How long do COVID-19 patients remain infectious?
Research suggests patients became infectious 2 days before symptom onset and can only spread the infection for up to 7-10 days after showing the symptoms.
On May 23, the Chapter of Infectious Disease Physicians, Academy of Medicine, and the National Center for Infectious Diseases (NCID), Singapore released a joint paper suggesting that COVID-19 patients do not have any viable virus after the second week (despite showing positive results in RT-PCR test) of illness.
According to the paper, the patients became infectious (able to spread the infection) two days before symptom onset and can only spread the infection for up to 7-10 days after showing the symptoms. It also suggests that viral replication drops quickly after the first week and after the second week, the patient does not have enough viable virus (that can cause infection in a healthy person) left in their body.
The paper's suggestion has been given after collecting the epidemiological, microbiological and clinical data from coronavirus patients. It goes on to state that the discharge criteria of COVID-19 patients may be changed based on these findings. Instead, more people should be identified in the early phase of the disease to limit the spread of the infection.
However, one uncertainty in the path of the suggestion is that we don’t know for sure yet how much of a role do asymptomatic patients play in the spread the infection.
As per the NCID, Singapore, data collected from about 766 patients in the country showed that the nasopharyngeal swabs of 30% of the patients showed negative RT-PCR results by day 15, about 68% of the patients by day 21, 88% by day 28 and 95% by day 33.
However, the presence of viral RNA does not necessarily mean that the virus is still viable or the person can spread the infection. Another study of 73 COVID-19 patients in Singapore has found that when the cycle threshold value of RT-PCR was above 30, the patients did not have any viable virus in their body. Also, none of the patients showed viable virus after day 11 of their illness. Cycle threshold in PCR is the number of PCR cycles required for generating a detectable amount of copies of the nucleic acid (DNA/RNA) sequence. The higher the DNA/RNA in a sample, the lesser the cycle threshold value.
A study conducted in Germany also supports the same data where no viable virus was found in the throat and lung samples from a group of patients after day 8 of the illness. All these patients still had an otherwise high viral load as detected by the RT-PCR test. Subgenomic RNA indicating viral replication also decreased gradually in sputum samples from day 11 and it was not detected in throat swabs after day 5.
As per the paper, many countries have reported people being COVID-19 positive after being tested negative for the disease previously. Since we don’t know much about the infectivity of COVID-19 patients so far, these results have led to speculations that a person may become a carrier of the disease or can get recurrent infections. A carrier is someone who does not have symptoms themselves but can spread the disease to healthy people.
However, four such patients in China and 180 in Korea have shown no presence of the live virus. A review by the Korean CDC had earlier mentioned that these patients had unviable RNA from dead viruses.
Epidemiological studies deal with the distribution and incidence of diseases in a population.
The paper mentioned the following epidemiological findings:
- A study including 100 COVID-19 patients and 2,761 of their close contacts had found that in at least 22 direct contacts and patient pairs, the contacts had got the disease while the patient was in the first 5 days of his/her illness. The rest of the contacts had been after the 5 days mark and had not got the disease.
- Another study conducted on 77 first and secondary patient pairs in Hong Kong estimated that the COVID-19 patients become infectious 2.4 days before symptom onset, are most infectious by day 3 and the infectiousness quickly declined after day 7.
For more information, read our article on Recovery time for COVID-19.
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