Scientists predict how the novel coronavirus spreads in a typical household

By Dr. Liji Thomas, MDApr 17 2020

The novel coronavirus has been spreading for over four months now, with global cases crossing 2.15 million and over 144,000 deaths. Termed the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), it causes a respiratory illness similar to those responsible for SARS and the common cold. SARS-CoV-2 spreads through droplets released by the infected through coughing and sneezing. Though most people recover without medical aid, the elderly, along with those who suffer from preexisting cardiovascular or respiratory illnesses, are at risk for severe lung injury or even death.

Novel Coronavirus SARS-CoV-2 Colorized scanning electron micrograph of an apoptotic cell (blue) infected with SARS-COV-2 virus particles (red), isolated from a patient sample. Image captured at the NIAID Integrated Research Facility (IRF) in Fort Detrick, Maryland. Credit: NIAID

Finding out how the virus spreads

A team of scientists recently carried out a study on the transmissibility of the virus in households. Simply put, they’re trying to predict how the virus spreads in a typical family: if one person gets sick, how many others get sick? How sick? How long would it take the family to recover? These are the practical questions this study tries to answer. Using a comprehensive list of contacts traced in China’s Guangzhou province, they estimated two things: the effective reproductive number and the secondary attack rate. The research is published on MedRxiv and has not yet been peer-reviewed.

The effective reproductive number is one of the basic concepts of the epidemiology of infectious diseases. Epidemiology refers to the study of diseases, their incidence and distribution, and possibly how they can be controlled. The effective reproductive number for a specific set of people (in this case, an average household) is the number of people that would get sick if one person in the set was infected. This is one of the most important factors in modeling the spread of the virus.

The secondary attack rate is a similar idea: it’s the next link in the chain, which can tell us how the virus spreads in a household. Secondary attack rate, or SAR, is obtained by finding the number of new cases in the population and dividing by the total number of people at risk: in other words, it is the ratio of the number of people that actually contract the virus, out of the population at risk.

How does age affect transmissibility?

The study also estimated the effect of age on transmissibility. At this point, it’s common knowledge that the elderly are at higher risk from the SARS-CoV-2 virus. This could be due to multiple factors. For one, elderly people often experience a weakening of the immune system, making them more susceptible to viruses and infections.

That’s not all, though: a mature immune system, as found in older people, is more likely to react violently to infection, giving rise to a phenomenon called a cytokine storm. This is when the immune system overreacts, flooding the body with defensive chemicals. Cytokine storms can lead to organ failure.

Again, older people are more likely to have one or more already existing medical conditions that put them at higher risk for disease and death already. The combination of these factors results in a greater risk of severe or fatal illness in the elderly population as a result of COVID-19.

How does the infectivity change over the incubation period?

Another factor that this study worked on to estimate the effects of was the infectivity of the virus during its incubation period. Unlike the Ebola virus, which manifests symptoms within a relatively short period after infections, SARS-CoV-2 can lie “dormant” within a patient for up to two weeks.

During this period, the patient will display few or no symptoms, but can still transmit the virus to others. The infectivity of a pathogen is its ability to enter an individual and cause illness. This study estimates the infectivity of the coronavirus during its incubation period.

What was the focus of the study?

The current study was focused on estimating the risk of transmissibility in a quantitative manner. The team traced 195 clusters with 212 primary cases, 137 non-primary cases (cases which were infected by primary cases), and 1938 close contacts who escaped infection.

The results of the study showed that an accurate estimate of the household secondary attack rate was 13.8%: in other words, about 14% of the people at risk in a household end up contracting the virus.  This estimate is only valid if the household contacts include all close relatives.

Restricting the set of household contacts to only those at the same residential address as the primary infected individual means that the SAR works out to 19.3%.

This assumes an average incubation period of four days and a maximum infectious period (the period within which an infected individual can transmit the virus to others) of 13 days. The odds of infection among the elderly (individuals over 60 years old) were 3.85 times greater than those among children (those less than 20 years old).

There was no difference in the risk of infection between genders, and the coronavirus was found to be as infectious in its incubation period as after symptoms had manifested.

Summing up

The study found that on average, a SARS-CoV-2 case infects 48% of close contacts. Without isolation, this number increases to 62%. The effective reproductive number from their dataset in Guangzhou decreased from above 1 to below 0.5 in about a week.

The team concluded that the coronavirus is more transmissible in households than SARS-CoV and MERS-CoV and that those above sixty are at the most risk. This confirms the need for high-risk populations such as the elderly to practice self-isolation to minimize the impact of the virus.

Important Notice

medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.