Data | When are COVID-19 patients most likely to infect others?

People wearing face masks walk inside a subway station during morning rush hour in Beijing, as the spread of the novel coronavirus disease (COVID-19) continues in China on April 14, 2020.   | Photo Credit: REUTERS

Patient data from southeast Asian nations show that a significant share of COVID-19 patients passed on the infection to others when they were not showing any symptoms (during the pre-symptomatic period).

Also, the ability of a symptomatic patient to transmit the infection drops to very low levels just a week after the onset of symptoms. However, they may continue to test positive for the disease.

Infectious period

In China, 77 infector-infectee pairs (or primary case-secondary case pairs) were studied to measure the infectious period (IP) of the primary COVID-19 cases. Infectious period is the time taken for the primary case to transmit the infection to the secondary case(s).

When does infectiousness peak?

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−3−2−1012345678 image/svg+xml 0 10 20 Density (%) Days after symptom onset Infection period started 2.3 days before the onset of symptoms in the primary case Infectiousness peaked at 0.7 days before the onset of symptoms The study found that 44% of secondary cases were infected during the presymptomatic stage of the primary cases Infectiousness declined to very low levels within 7 days Symptoms onset Infectiousness (%) Days after symptom onset -3 -2 -1 0 1 2 3 4 5 6 7 8

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The above chart shows that isolating only symptomatic patients will not be effective. Contact tracing secondary infections well before symptom onset is important.

Also read: Data | Is India undercounting its COVID-19 deaths?

Contact history

In Taiwan, 100 primary patients and their 219 family contacts were studied. Only 11 contacts turned positive. All the 11 were among the 176 who contacted the primary patients within the first five days of their symptom onset.

The crucial period

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image/svg+xml None of the 43 who came into contact with the primary patients after 5 days was infected Clinical attack rate, % <00-34-56-78-9>9 12 8 0 4 Counts Days from onset to exposure Count 120 90 60 30 0 Days after symptom onset <0 0-3 4-5 6-7 8-9 >9 % of contacts who turned +ve (right axis) Close contacts Contacts who turned +ve 0

The above chart shows that after just 5 days of onset of symptoms in primary contacts, the transmission of infection approaches 0%.

Negative result

Analysis of 766 COVID-19 patients in Singapore shows that 30% tested negative (by nasopharyngeal swab) by day 15, 68% by day 21, 88% by day 28 and 95% by day 33. Thus, some patients may continue to test positive even after a month of illness.

How long before patients test negative?

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image/svg+xml 95 70 45 20 15 21 28 33 Days since illness onset % tested negative

However, the first two graphs show that their ability to transmit the infection drops to very low levels a week after the onset of symptoms.

Last word

As the patients’ ability to transmit the virus drops to low levels after a week of the onset of symptoms, they may be discharged early. This will significantly destress the hospital sector.

Testing and isolating pre-symptomatic patients sooner may significantly reduce the disease spread.


He, X., Lau, E.H.Y., Wu, P. et al. Temporal dynamics in viral shedding and transmissibility of COVID-19. Nat Med 26, 672–675 (2020)

Cheng H, Jian S, Liu D, et al. Contact Tracing Assessment of COVID-19 Transmission Dynamics in Taiwan and Risk at Different Exposure Periods Before and After Symptom Onset. JAMA Intern Med. Published online May 01, 2020

Position Statement from the National Centre for Infectious Diseases and the Chapter of Infectious Disease Physicians, Academy of Medicine, Singapore – 23 May 2020

Also read: COVID-19 | State-wise tracker for coronavirus cases, deaths and testing rates

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Printable version | May 11, 2021 11:07:47 AM |

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