Only 5 per cent of people in close contact with a Covid-19-infected person actually catch the disease, says new study.

20 July 2020 By Paul Martin

The infectiousness of Covid-19 is surprisingly low, a Harvard University medical scientist has written in a prestigious health publication, the Journal of the American Medical Association.

He says the disease will only spread to 3 per cent of health workers who come into prolonged contact with a person who was already infectious with Covid-19.

“First, the reproduction number for COVID-19 before measures were taken to mitigate its spread was estimated to be about 2.5, meaning that each person with COVID-19 infected an average of 2 to 3 other people. This reproduction number is similar to influenza and quite different from that of viruses that are well known to spread via aerosols such as measles, which has a reproduction number closer to 18.

“Considering that most people with COVID-19 are contagious for about 1 week, a reproduction number of 2 to 3 is quite small given the large number of interactions, crowds, and personal contacts that most people have under normal circumstances within a 7-day period. Either the amount of SARS-CoV-2 required to cause infection is much larger than measles or aerosols are not the dominant mode of transmission.

“Similarly, the secondary attack rate for SARS-CoV-2 is low. Case series that have evaluated close contacts of patients with confirmed COVID-19 have reported that only about 5% of contacts become infected. However, even this low attack rate is not spread evenly among close contacts but varies depending on the duration and intensity of contact.

“The risk is highest among household members, in whom transmission rates range between 10% and 40%.24 Close but less sustained contact such as sharing a meal is associated with a secondary attack rate of about 7%, whereas passing interactions among people shopping is associated with a secondary attack rate of 0.6%.4

“The secondary attack rate among health care workers who unknowingly care for a patient with COVID-19 while wearing face masks alone or not using any personal protective equipment is also low; transmission studies suggest less than 3% (and the few health care worker infections that were documented in these transmission studies were associated with aerosol-generating procedures or prolonged exposures with inconsistent use of face masks).5,6

The writer is Dr Michael Klompas, of the Harvard Medical School and Harvard Pilgrim Health Care Institute. He is a professor in the Department of Population Medicine. [His research, quoting other studies, was debunking the likelihood that people catch Covid-19 from tiny droplets suspended for hours in the air, rather than from heavier droplets that are breathed in before they can fall to ground within around a 6-feet radius when a person coughs or sneezes.]


2. Cheng  HY, Jian  SW, Liu  DP, Ng  TC, Huang  WT, Lin  HH; Taiwan COVID-19 Outbreak Investigation Team.  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 1, 2020. doi:10.1001/jamainternmed.2020.2020

3. Rosenberg  ES, Dufort  EM, Blog  DS,  et al; New York State Coronavirus 2019 Response Team.  COVID-19 testing, epidemic features, hospital outcomes, and household prevalence, New York State-March 2020.   Clin Infect Dis. Published online May 8, 2020.

4. Chen  Y, Wang  AH, Yi  B,  et al.  Epidemiological characteristics of infection in COVID-19 close contacts in Ningbo.  Article in Chinese.  Zhonghua Liu Xing Bing Xue Za Zhi. 2020;41(5):667-671.

5.Heinzerling  A, Stuckey  MJ, Scheuer  T,  et al.  Transmission of COVID-19 to health care personnel during exposures to a hospitalized patient—Solano County, California, February 2020.   MMWR Morb Mortal Wkly Rep. 2020;69(15):472-476. doi:10.15585/mmwr.mm6915e5PubMedGoogle ScholarCrossref

6.Ng  K, Poon  BH, Kiat Puar  TH,  et al.  COVID-19 and the risk to health care workers: a case report.   Ann Intern Med. 2020;172(11):766-767. doi:10.7326/L20-0175