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At the start of the COVID-19 pandemic, there were many big questions about the underlying properties of SARS-CoV-2: how fast it spread, could it spread from asymptomatic people, what was the typical death rate, and so on. We quickly started getting answers on some of these, but they were all flawed in various ways. We could track all cases in controlled environments, such as a cruise ship or aircraft carrier, but these would likely not reflect the spread of the virus in more typical communities. Or, we could track things in real-world communities, but that approach would be far less certain than capturing all cases.
Over time, we’ve gotten a lot of flawed records, but we’ve started building consensus on them. The latest example of this, a document describing the contact traceability of all cases that originated in Hunan, China, provides another set of measures of the virus’s behavior and our attempts to control the infection. Documents like this helped build consensus on some of the key characteristics of things like asymptomatic spread and the impact of contact tracking, so we thought it was a good opportunity to step back and look at this latest release.
Track all cases
The new work, carried out by an international team of researchers, focuses on the spread of SARS-CoV-2 in Hunan province during the first outbreak since its origins in neighboring Hubei. During the study period, health authorities began by identifying cases primarily based on symptoms, then moved on to a massive contact tracing effort and aggressive isolation policies. These efforts stopped the outbreak in early March. And, thanks to them, we have very detailed information on viral cases: 1,178 infected individuals, 15,648 other people they came into contact with, and a total of nearly 20,000 potential exposure events.
(Some additional cases occurred in Hunan during this period due to people getting infected elsewhere and then traveling to the province, but these are not considered in the analysis.)
The good news: The researchers found that exposure in the healthcare setting was very low risk. This indicates that, as long as sufficient protective equipment is available, we can put procedures in place that minimize the spread of the virus to healthcare workers.
The highest risk was, not surprisingly, among those who shared a family, followed by members of the extended family. The risk here actually increased with the adoption of isolation and social distancing orders, as this forced people to spend more time indoors with infected people. This heightened risk occurred despite the fact that China adopted a policy in which people known to be infected were taken to dedicated isolation hospitals. Social and community contacts were intermediate-level risks.
The ability to quickly trace and isolate cases has also altered other aspects of the spread of the virus. In the absence of any public health intervention, the peak of infectivity – the midpoint after infection where someone was most likely to transmit the virus – was about 5.3 days. This is around the same time that the first symptoms become evident. (Both of these numbers are consistent with many additional studies elsewhere.)
But once efficient contact tracing was in place, infected people typically went into isolation first. As a result, most people passed on infections earlier, before being isolated. (So there has been a shift in infection patterns even though nothing about the virus has changed.) So while they found that nearly two-thirds of people passed on the virus before symptoms appeared, some of these are a health product publishes measures taken.
Who infected who?
It was clear that not all individuals were equally infectious, something that had previously been seen. The authors estimate that 80% of infections could be traced back to only 15% of individuals, although again that number would have been impacted by contact tracing and isolation from Hunan, which would have prevented infections from many of those infected. in the later stages of the epidemic. This study does not shed light on whether it is because some people’s infections are more likely to spread for biological reasons or because some individuals have participated in “superspreader” events in which environmental conditions have allowed high levels of infection.
One thing the researchers noted was that those under the age of 12 were as likely to transmit the virus as young adults. But they had seen that fewer children in that age group had been infected in the first place. The role of children in leading outbreaks has been the subject of some controversy. It is clear that young children are less likely to experience symptoms, but it is less clear whether this was partly explained by the fact that they are less likely to contract the virus in the first place. Obviously, this is something we will seek more certainty about, as it will help shape education policy.
Finally, the researchers looked at one of the basic measures of infectivity, the viral reproduction number (called R-naught), which measures how many infections on average result from each infected individual. They produced a reference number similar to that seen in other cases: 2.19. Anything above one means the pandemic will grow, so such a high number is why SARS-CoV-2 has been so dangerous.
All China interventions lowered this value, with the start of contact tracing leading it to 1.5 and aggressive contact tracing and isolation leading it to 1.01. Although the latest figure represents a case where the size of the epidemic is barely growing, it is still not enough to completely stop the circulation of the virus. “In practice, epidemic control is unrealistic if case isolation and quarantine of close contacts are the only measures in place,” the authors conclude.
Fortunately, they’re not the only tools we have; cite policies such as increased teleworking, reduced restaurant operations and other service activities, and increased use of face masks as options that would add to the contact tracing and isolation effect. And, because the margins are so close to the start, we wouldn’t even need high levels of adoption to make a sufficient difference.
Of course, all that happened in a country where the government could guarantee any political decision was taken and the cases were low enough to make contact tracing effective. This is certainly not the case in the United States, so it is likely that more parallel efforts will be needed before contact tracing can be effective.
Overall, however, it is crucial not to place too much emphasis on the results of an article; the strength of this is that it largely supports data obtained from a variety of other sources. As the consensus built from these findings grows stronger, we will be in a better position to tailor public health policies to the properties of the virus.
Science, 2020. DOI: 10.1126 / science.abe2424 (About DOI).
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