I honestly don’t know what to make of this study in Nature. Here’s the abstract:
Stringent COVID-19 control measures were imposed in Wuhan between January 23 and April 8, 2020. Estimates of the prevalence of infection following the release of restrictions could inform post-lockdown pandemic management. Here, we describe a city-wide SARS-CoV-2 nucleic acid screening programme between May 14 and June 1, 2020 in Wuhan. All city residents aged six years or older were eligible and 9,899,828 (92.9%) participated. No new symptomatic cases and 300 asymptomatic cases (detection rate 0.303/10,000, 95% CI 0.270–0.339/10,000) were identified. There were no positive tests amongst 1,174 close contacts of asymptomatic cases. 107 of 34,424 previously recovered COVID-19 patients tested positive again (re-positive rate 0.31%, 95% CI 0.423–0.574%). The prevalence of SARS-CoV-2 infection in Wuhan was therefore very low five to eight weeks after the end of lockdown.
and
Virus cultures were negative for all asymptomatic positive and repositive cases, indicating no “viable virus†in positive cases detected in this study.
while from the discussion section:
This study has several limitations that need to be discussed. First, this was a cross-sectional screening programme, and we are unable to assess the changes over time in asymptomatic positive and reoperative results. Second, although a positive result of nucleic acid testing reveals the existence of the viral RNAs, some false negative results were likely to have occurred, in particular due to the relatively low level of virus loads in asymptomatic infected individuals, inadequate collection of samples, and limited accuracy of the testing technology13. Although the screening programme provided no direct evidence on the sensitivity and specificity of the testing method used, a meta-analysis reported a pooled sensitivity of 73% (95% CI 68–78%) for nasopharayngeal and throat swab testing of COVID-1914. Testing kits used in the screening programme were publicly purchased by the government and these kits have been widely used in China and other countries.
I also have a question: how did they define “asymptomatic”?
I have reservations about this study not for the least because of its origins. However, if true, doesn’t it suggest that the approach taken by governors in most states has been wrong? The necessary mitigation would appear to be mandatory quarantining of the symptomatic. And that wearing masks and social distancing is most effective when practiced by the symptomatic. While distasteful is that not a lot more manageable with fewer adverse consequences than shuttering businesses?
If you don’t like that Wuhan study, another one dropped yesterday:
https://science.sciencemag.org/content/early/2020/11/23/science.abe2424
1. “In addition, susceptibility to infection (defined as the risk of infection given a contact with primary case) by age: children aged 0-12 years are significantly less susceptible than individuals 26-64 years (odds ratio 0.41, 95% CI 0.26 to 0.63); while patients older than 65 years are significantly more susceptible (odds ratio 1.39, 95% CI 1.02 to 1.91).”
2. “In contrast, we find no statistical support for age difference in infectivity (fig. S3A).”
3. “We estimate that 63.4% (95% CI, 60.2% to 67.2%) of all transmission events occur before symptom onset, which is comparable with findings from other studies (6–8, 10–13, 18, 30, 31). However, these estimates are impacted by the intensity of interventions; in Hunan, isolation and quarantine were in place throughout the epidemic.”
4. “Notably, we find that SARS-CoV-2 infectiousness peaks slightly before symptom onset (-0.1 days on average), with 87% of the overall infectiousness concentrated within ±5 days of symptom onset and 53% of the overall infectiousness in the pre-symptomatic phase (Fig. 3E).”
5. “Our data support that case isolation and quarantine of close contacts are effective in reducing SARS-CoV-2 transmission, especially if these interventions occur early in the infection. To achieve epidemic control, however, these interventions need to be layered with additional population-level measures, including increased teleworking, reduced operation in the service industry, or broader adoption of face masks”
I don’t see how #3 and #4 are compatible with the study linked in the post.
I think the study in the OP is purely asymptomatic, while the study I link to are people that eventually showed symptoms. If you never have symptoms, you’ve never shed enough viruses to infect anyone. If you do ultimately develop symptoms, you started shedding a lot of viruses before symptom onset.
The time of policies is difficult, particularly if people don’t know whether they are infected or capable of infecting other people. Initially, China responded to symptom surveillance, which meant that the median time from onset to isolation was 5.4 days. Since infectiousness is centered five days from symptom onset, this was not very effective. Contact tracing led to a shortening of the median time from onset to isolation to -0.1 days, which is the greatest point of infectiousness, but that was still insufficient to R_0 below 1.00 (though it was estimated to be 1.01). That must be because still a lot of infectious are transmitted pre presymptomatically, and also 4.3% of transmission occurred after SARS-CoV-2 patients were isolated.
Also, the study I linked to does not “evaluate the risk of transmission in schools, workplaces, conferences, prisons, or factories, as no contacts in these settings were reported in the Hunan dataset.”
That’s part of the point of my question. Everybody is asymptomatic until they’re symptomatic. Without rigorous definitions it’s all sophistry.
The 2 studies are looking at very different situations.
The OP study was conducted in a period where the number of people actively infected with COVID was 0 or very likely close to 0 (from other known benchmarks like hospital admissions, people forcibly quarantined).
PCR is going to give a deceptive data because some infected people have inert viral RNA circulating in their bodies months after infection. We have seen case reports of the sort all year, leading to sensational stories about whether reinfection was occurring. It is plausible (likely) the OP study just detected a lot of those people.
The 2nd study is a study of all cases in Wuhan, through the active outbreak.
Apparent difference in fatalities between Central Europe and East Asia due to SARS-COV-2 and COVID-19: Four hypotheses for possible explanation:
“1) Differences in social behaviors and cultures of people in the two regions; 2) Possible outbreak of virulent viruses in Central Europe due to multiple viral infection, and the involvement of immuno-virological factors associated with it, 3) Possibility of corona resistance gene mutation occurring among East Asians as a result of long-term co-evolution of virus and host, and 4) possible involvement of hygienic factors.”
https://www.sciencedirect.com/science/article/pii/S0306987720314912
Note that Central Europe has an odd definition here, it is Italy, Spain, France, Germany and UK. Hypothesis #2 is a mutation theory, which theorizes that multiple contemporaneous infections as would have occurred in the early Italian outbreak could cause selection for the most virulent strand. Hypothesis #3 has received support from a recent genomic study that found 25,000 years ago an ancient corona virus drove East Asian adaptation; but this is so long ago that it would impact Native Americans and Pacific Islanders as well. Hypothesis #4 is actually a theory based upon East Asians having advanced immunological benefits from prior exposure to certain viruses (a poor hygiene model) or alternatively to unique vaccines like Japanese encephalitis (a high hygiene model).
There’s been enough travel between Europe and East Asia since the spring that East Asia should have been overwhelmed if #2 was the dominant factor.
Given we don’t see evidence of #3, that leaves #1 and #4.
And how do you distinguish between #1 and #4? Possibly just one hypothesis there.
Another possibility: the strain of the virus that prevailed in China, Japan, and Taiwan was different than the one that took hold in Italy, France, Spain, etc. (and the U. S.). That’s being hotly debated.
That’s #2. I pointed out in the comment above the evidence points away from it.
i.e. in the argument on what is more responsible for the difference in the pandemic between regions; the virus or humans; the evidence leans towards humans.
That’s not to say that factor in humans is something that is under conscious control. There are #3 genes (not controllable); #4 “immune ecosystem” (semi controllable); #1 culture and behaviors (semi controllable).
I think that the Chinese in particular should be careful about pushing too hard on #3. I don’t believe I can think of a better reason for quarantining the whole country of China. “Nuke the site from orbit; it’s the only way to be sure”.
“And that wearing masks and social distancing is most effective when practiced by the symptomatic.”
PD’s study is one of several now that have documented you are probably most infectious right before you turn symptomatic. So there isn’t a good way to predict who is asymptomatic and will stay way and who will become symptomatic.
Seems to me the lesson here is that a very harsh lockdown early can pretty successfully stop things, but it needs to be so harsh it will not be acceptable in the US.
Steve
Arstechnica’s summary of the study has a key line. Emphasis is mine.
https://arstechnica.com/science/2020/11/what-we-can-learn-from-contact-tracing-an-entire-province/
“The highest risk was, not surprisingly, among those sharing a household, followed by extended family members. The risk here actually went up as social distancing and isolation orders were put in place, as this forced people to spend more time in enclosed spaces with infected people. This heightened risk occurred despite the fact that China adopted a policy where people known to be infected were brought to dedicated isolation hospitals. Social and community contacts were intermediate-level risks.”
As the study summarizes, the Chinese found the key to controlling the outbreak was very strict isolation of known cases and their households. Which actually is what was suggested in the OP.