This means contact tracing needs to capture more people to be successful

It is no longer a question of whether there is human-to-human transmission of Covid-19, but what is the peak time for transmission — when people are symptomatic or before they show any symptoms?

A small report from China suggested that the peak time that people are most contagious is at or before symptom onset, and they noted that potential transmission 2-3 days before symptom onset needs to be considered when undertaking contact tracing.

The study, published in Nature Medicine, was conducted by Xi He, MD, from Guangzhou Eighth People’s Hospital, Guangzhou Medical University, Guangzhou, China, and colleagues, who wrote, “compared clinical data on virus shedding with separate epidemiological data on incubation periods and serial intervals between cases in transmission chains, to draw inferences on infectiousness profiles.”

The researchers collected 414 throat swabs from 94 patients admitted to the Eighth People’s Hospital. This procedure began at symptom onset and up to 32 days after onset.

“We detected high viral loads soon after symptom onset, which then gradually decreased towards the detection limit at about day 21,” He and colleagues wrote. They did not detect a significant difference in viral loads by sex, age, or disease severity in the patients (median age 47, half male, and 66% had moderate symptoms — fever and/or respiratory symptoms and pneumonia).

“Separately, based on 77 transmission pairs obtained from publicly available sources within and outside mainland China, the serial interval was estimated to have a mean of 5.8 days (95% confidence interval (CI), 4.8–6.8 days) and a median of 5.2 days (95% CI, 4.1–6.4 days) based on a fitted gamma distribution, with 7.6% negative serial intervals,” He and colleagues wrote. “Assuming an incubation period distribution of mean 5.2 days from a separate study of early COVID-19 cases, we inferred that infectiousness started from 12.3 days (95% CI, 5.9–17.0 days) before symptom onset and peaked at symptom onset (95% CI, -0.9-0.9 days)… The estimated proportion of presymptomatic transmission (area under the curve) was 44% (95% CI, 25–69%).”

They also noted that infectiousness waned within 7 days.

In a sensitivity analysis using the same assumptions but holding 1-7 days before symptom onset as the constant for the start of infectiousness, peak infectiousness was 0-2 days prior to symptom onset, “and the proportion of presymptomatic transmission ranged from 46% to 55%.”

Further, assuming that infectiousness starts prior to symptom onset, a simulation model showed the intervals would be larger: “Given the 7.6% negative serial intervals estimated from the infector–infectee paired data, start of infectiousness at least 2 days before onset and peak infectiousness at 2 days before to 1 day after onset would be most consistent with this observed proportion,” He and colleagues wrote.

The study authors pointed out that most of the patients were put in isolation once their symptoms appeared, so transmission after symptoms presented was mostly prevented.

He and colleagues noted that, in Singapore and Tianjin, there are higher proportions of presymptomatic transmission — 48% and 62%, respectively — likely due to active case findings, and quick quarantine, which helps reduce secondary transmission.

Interestingly, the study authors noted that two studies show that there is a decrease in viral load after symptom onset. While they pointed to a study from Wuhan that showed that SARS-CoV-2 was detected for a median of 20 days and in survivors up to 37 days, “infectiousness may decline significantly 8 days after symptom onset, as live virus could no longer be cultured (according to Wölfel and colleagues).”

“Together, these results support our findings that the infectiousness profile may more closely resemble that of influenza than of SARS, although we did not have data on viral shedding before symptom onset,” they wrote. “Our results are also supported by reports of asymptomatic and presymptomatic transmission”

The value of these findings is for contact tracing, which won’t be successful, He and colleagues noted, if more than 30% of transmission occurs prior to symptom onset, “Unless >90% of the contacts can be traced.”

  1. Covid-19 appears to be most contagious 2-3 days prior to symptom onset.
  2. Given this, the study authors note that transmission of Covid-19 is more like the flu than SARS.

Candace Hoffmann, Managing Editor, BreakingMED™

He disclosed no relevant relationships.


Cat ID: 125

Topic ID: 79,125,254,930,287,500,503,728,932,730,933,125,520,926,192,927,151,928,925,934


He X, et al “Temporal dynamics in viral shedding and transmissibility of Covid-19” Nat Med 2020; DOI: 10.1038/s41591-020-0869-5.