Enlarge /. Students' temperature is measured at Daowu Middle School in China's Hunan Province. This is part of the measures to limit the spread of the coronavirus.
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At the beginning of the COVID-19 pandemic, there were many big questions about the basic properties of SARS-CoV-2: how quickly did it spread, could it spread from asymptomatic people, what was the typical death rate and so on. We got quick answers to some of these questions, but they were all imperfect in various ways. We could all track cases in controlled settings like a cruise ship or an aircraft carrier, but these would likely not reflect the spread of the virus in more typical communities. Or we could keep track of things in real communities, but that approach would be far less secure to cover all cases.
We have received many incomplete records over time, but we have started to reach consensus from them. The most recent example of this – an article describing contact tracing of all cases originating in Hunan, China – includes further measures on virus behavior and our attempts to control infections. Papers like this one have helped build consensus on some of the key features of things like asymptomatic spread and the effects of contact tracing. So we thought it would be a good opportunity to take a step back and check out this latest version.
Keep track of all cases
New work by an international team of researchers focuses on the spread of SARS-CoV-2 in Hunan Province during the first outbreak since it started in nearby Hubei. During the study period, health officials initially identified cases largely based on symptoms and then switched to massive contact tracing efforts and aggressive isolation guidelines. These efforts completed the outbreak by early March. And thanks to them, we have very detailed information on virus cases: 1,178 infected people, another 15,648 people they came into contact with, and a total of almost 20,000 potential exposure events.
(Some additional cases occurred in Hunan during this time due to people being infected elsewhere and then traveling to the province, but these are not included in the analysis.)
Good news: the researchers found that health care exposure was a very low risk. This indicates that as long as adequate protective equipment is available, we can put procedures in place to minimize the spread of viruses to healthcare workers.
Unsurprisingly, those who share a household were at greatest risk, followed by extended family members. The risk here actually increased when social distancing and isolation orders were introduced, as this forced people to spend more time indoors with infected people. This increased risk occurred despite the fact that China had a policy of transferring people known to be infected to special isolation hospitals. Social and communal contacts were medium-level risks.
The ability to quickly track down and isolate cases also changed other aspects of the virus's spread. In the absence of any public health interventions, the peak of infectivity – the average time after infection when someone was most likely to pass the virus on – was around 5.3 days. This is around the same time that symptoms first appear. (These two numbers are in line with many additional studies elsewhere.)
However, once efficient contact tracing was put in place, infected people were typically isolated earlier. As a result, most people passed infections earlier before they were isolated. (So there was a change in infection patterns, although nothing about the virus changed.) While they found that nearly two-thirds of people transmitted the virus before symptoms appeared, some of it is a product of public health measurements .
Who infected whom?
It was clear that not all individuals were equally infectious from what had been seen before. The authors estimate that 80 percent of the infections could be traced back to just 15 percent of the people, although that number would in turn be influenced by Hunan's contact tracing and isolation, which would have prevented infections from many of those infected in the later stages of the outbreak. This study does not indicate whether this is due to the fact that some people's infections are more likely to spread for biological reasons, or whether some people have participated in "superspreader" events where environmental conditions allowed for high levels of infection.
One thing the researchers found was that people under the age of 12 are just as likely to pass the virus on as young adults. But they had seen that fewer children in this age group were infected at all. The role of children in driving breakouts has been the subject of some controversy. It's clear that young children experience fewer symptoms, but it was less clear whether this was partly explained by their less likely to ingest the virus at all. This is, of course, something that we will be looking for more security about as it will help set education policy.
Finally, the researchers examined one of the basic measures of infectivity, the viral reproductive number (called R-naught), which measures the average number of infections caused by each infected individual. They came with a similar base number as in other cases: 2.19. Anything above one means the pandemic will intensify. Such a high number is why SARS-CoV-2 was so dangerous.
All of China's interventions have lowered this value, with the start of contact tracing dropping to 1.5 and aggressive contact tracing and isolation dropping to 1.01. While that last number represents a case where the size of the outbreak barely increases, it is still not enough to completely end the cycle of the virus. "In practice, disease control is unrealistic if there is only case isolation and close quarantine contacts," the authors conclude.
Fortunately, they're not the only tools we have. They cite guidelines like more teleworking, less running of restaurants and other service companies, and greater use of face masks as options that would increase the effect of contact tracing and isolation. And since the margins are so close to begin with, we don't even need a high level of acceptance to make a sufficient difference.
Of course, it all happened in a country where the government could make sure policy decisions were made, and the cases were so small that contact tracing could be effective. This is certainly not the case in the US, so it is likely that several parallel efforts will be required before full contact tracing can be effective at all.
Overall, however, it is important not to overemphasize the results of a paper. The strength of this model is that it largely supports data drawn from a variety of other sources. The stronger the consensus that emerges from these findings, the better we can adapt public health guidelines to the characteristics of the virus.
Science, 2020. DOI: 10.1126 / science.abe2424 (About DOIs).