#14,668
Whenever we see an outbreak of an infectious disease, the reported number of cases is always assumed to be a subset of the actual number of cases in a community. Over the years we've looked at dozens of attempts to estimate the true burden of these outbreaks, including:
CDC Updates Estimates Of Infections, Hospitalizations, and Deaths From H1N1
CID Journal: Estimates Of Human Infection From H3N2v (Jul 2011-Apr 2012).
BMC: Estimating The Transmission Potential Of H7N9
EID Journal: Estimation of Severe MERS Cases in the Middle East, 2012–2016
In every one of these studies, the actual number of cases was estimated to be anywhere from 10 to more than 100 times greater than what was being picked up by early surveillance.
In 2015, when Saudi Arabia had recorded fewer than 1200 MERS cases, a seroprevalence study (see Presence of Middle East respiratory syndrome coronavirus antibodies in Saudi Arabia: a nationwide, cross-sectional, serological study by Drosten & Memish et al.,) suggested nearly 45,000 might have been infected.
First, this summary from their twitter feed (tweets combined).
MRC Centre for Global Infectious Disease Analysis
@MRC_Outbreak ·1h
Size of Wuhan outbreak of a novel #coronavirus estimated from the three cases detected outside China: Likely to be over 1000 cases.
@imperialcollege @mrc_outbreaks report released today
https://imperial.ac.uk/mrc-global-infectious-disease-analysis/news--wuhan-coronavirus/
Summary of report (tweet 2 of 4)
We calculate, based on flight and population data, that there is only a 1 in 574 chance that a person infected in Wuhan would travel overseas before they sought medical care.
Summary of report (tweet 3 of 4)
This implies there might have been over 1700 (3 x 574) cases in Wuhan so far. There are many unknowns, meaning the uncertainty range around this estimate goes from 190 cases to over 4000.
Summary of report (tweet 4 of 4)Some excerpts from the analysis (caution, higher maths ahead). Follow the link to read it in its entirety:
But the magnitude of these numbers suggests that substantial human to human transmission cannot be ruled out. Heightened surveillance, prompt information sharing and enhanced preparedness are recommended.
Estimating the potential total number of novel Coronavirus cases in Wuhan City, China
Natsuko Imai, Ilaria Dorigatti, Anne Cori, Steven Riley, Neil M. Ferguson
WHO Collaborating Centre for Infectious Disease Modelling
MRC Centre for Global Infectious Disease Analysis, J-IDEA, Imperial College London, UK
Correspondence: neil.ferguson@imperial.ac.uk
V2 (updated to include second Thai case)
Background
On the 31st December 2019, the World Health Organization (WHO) China Country Office was informed of cases of pneumonia of unknown aetiology in Wuhan City, Hubei Province, China [1]. A novel Coronavirus (2019-nCoV) related to the Middle Eastern Respiratory Syndrome virus and the Severe Acute Respiratory Syndrome virus has since been implicated [2].
As of 16th January 2020, 41 cases (including two deaths [3]) have been confirmed in Wuhan City with three confirmed cases in travellers detected in Thailand (2 cases) and Japan (1 case) [4–7]. Most cases have been epidemiologically linked to exposure at a seafood market in Wuhan which has been closed since 1 January 2020 in efforts to contain the outbreak. Although both travellers have a history of travel to Wuhan City, they did not visit the seafood market implicated in the other cases [2].
Using the number of cases detected outside China, it is possible (see Methods) to infer the number of clinically comparable cases within Wuhan City that may have occurred thus far.
Summary
We estimate that a total of 1,723 cases of 2019-nCoV in Wuhan City (95% CI: 427 – 4,471) had onset of symptoms by 12th January 2020 (the last reported onset date of any case).
This estimate is based on the following assumptions:
- Wuhan International Airport has a catchment population of 19 million individuals [1].
- There is a mean 10-day delay between infection and detection, comprising a 5-6 day incubation period [8,9] and a 4-5 day delay from symptom onset to detection/hospitalisation of a case (the cases detected in Thailand and Japan were hospitalised 3 and 7 days after onset, respectively) [4,10].
Caveats
- Total volume of international travel from Wuhan over the last two months has been 3,301 passengers per day. This estimate is derived from the 3,418 foreign passengers per day in the top 20 country destinations based on 2018 IATA data [11], and uses 2016 IATA data held by Imperial College to correct for the travel surge at Chinese New Year present in the latter data (which has not happened yet this year) and for travel to countries outside the top 20 destination list.
- We assume that outbound trip durations are long enough that an infected Wuhan resident travelling internationally will develop symptoms and be detected overseas, rather than being detected after returning to Wuhan. We also do not account for the fact that international visitors to Wuhan (such as the case who was detected in Japan) might be expected to have a shorter duration of exposure and thus a lower infection risk than residents. Accounting for either factor correctly requires additional data but would increase our estimate of the total number of cases.
- We estimate the potential number of symptomatic cases with disease severity of a level requiring hospitalisation (both the cases detected in Thailand and Japan were moderately severe). Our estimates do not include cases with mild or no symptoms.
- The incubation period of 2019-nCov is not known and has been approximated with the estimates obtained for MERS-CoV and SARS [8,9].
- We assume that international travel is independent of the risk of exposure to 2019n-CoV or of infection status. If zoonotic exposure was biased towards wealthier people, travel frequency may be correlated with exposure. Also, some travel might be causally linked to infection status (to seek healthcare overseas) or the infection status of contacts in Wuhan (this may apply to the case detected in Japan) [10]. Accounting for either association would increase the probability of a case travelling and therefore reduce our estimates of the total number of cases.
(Continue . . . )
Just as with estimated number of MERS-CoV and H7N9 cases, the higher estimated numbers don't necessarily presage a pandemic. But it does remind us that with any surveillance system, we are almost always just looking at the tip of the pyramid.
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Credit CDC |
