WHO Raises SARS Fatality Rate Estimates

WHO ^

[per loin]

7 May 2003

Case fatality ratio
WHO has today revised its initial estimates of the case fatality ratio of SARS. The revision is based on an analysis of the latest data from Canada, China, Hong Kong SAR, Singapore, and Viet Nam.

On the basis of more detailed and complete data, and more reliable methods, WHO now estimates that the case fatality ratio of SARS ranges from 0% to 50% depending on the age group affected, with an overall estimate of case fatality of 14% to 15%.

The likelihood of dying from SARS in a given area has been shown to depend on the profile of the cases, including the age group most affected and the presence of underlying disease. Based on data received by WHO to date, the case fatality ratio is estimated to be less than 1% in persons aged 24 years or younger, 6% in persons aged 25 to 44 years, 15% in persons aged 45 to 64 years, and greater than 50% in persons aged 65 years and older.

A case fatality ratio measures the proportion of all people with a disease who will die from the disease. In other words, it measures the likelihood that a disease will kill its host, and is thus an important indicator of the severity of a disease and its significance as a public health problem. The likelihood that a person will die of SARS could be influenced by factors related to the SARS virus, the route of exposure and dose (amount) of virus, personal factors such as age or the presence of another disease, and access to prompt medical care.

Many factors complicate efforts to calculate a case fatality ratio while an outbreak is still evolving. Deaths from SARS typically occur after several weeks of illness. Full recovery may take even longer. While an epidemic is still evolving, only some of the individuals affected by the disease will have died or recovered. Only at the end of an epidemic can an absolute value be calculated, taking into account total deaths, total recoveries and people lost to follow-up. Calculating case fatality as the number of deaths reported divided by the number of cases reported irrespective of the time elapsed since they became ill gives an underestimate of the true case fatality ratio.

One method of overcoming this difficulty is to calculate the case fatality ratio using only those cases whose final outcome – died or recovered – is known. However, this method, when applied before an outbreak is over, gives an overestimate because the average time from illness onset to death for SARS is shorter than the average time from illness onset to recovery.

With these methods, estimates of the case fatality ratio range from 11% to 17% in Hong Kong, from 13% to 15% in Singapore, from 15% to 19% in Canada, and from 5% to 13% in China.

A more accurate and unbiased estimation of case fatality for SARS can be obtained with a third method, survival analysis. This method relies on detailed individual data on the time from illness onset to death or full recovery, or time since illness onset for current cases. Using this method, WHO estimates that the case fatality ratio is 14% in Singapore and 15% in Hong Kong.

In Viet Nam, where SARS has been contained and measurement is more straightforward, case fatality was comparatively low, at 8%. One explanation for this is the large number of total cases that occurred in younger, previously healthy health care workers.

Incubation period
WHO has also reviewed estimates of the incubation period of SARS, using individual case data. On the basis of this review, WHO continues to conclude that the current best estimate of the maximum incubation period is 10 days.

The incubation period, which is the time from exposure to a causative agent to onset of disease, is particularly important as it forms the basis for many recommended control measures, including contact tracing and the duration of home isolation for contacts of probable SARS cases. Knowledge about the incubation period can also help physicians make diagnostic decisions about whether the presenting symptoms and clinical history of a patient point to SARS or to some other disease.

The incubation period can vary from one case to another according to the route by which the person was exposed, the dose of virus received, and other factors, including immune status. Estimates of the incubation period are further complicated by the fact that some patients have had opportunities for multiple exposures to the virus. The particular exposure that caused disease may prove impossible to determine. For these reasons, the most reliable estimates of the incubation period are based on a study of cases having a single documented exposure to a known case.

In today’s review, WHO has analysed the incubation periods of individuals with well-defined single-point exposures in Singapore, Canada, and Europe. Findings support the original estimate of 10 days as the maximum incubation period.

However, one recently published analysis of data from Hong Kong estimates a longer maximum incubation period in a group of 57 patients. This analysis, which may be significant and important for disease control, will be studied in more detail. The longer incubation period could reflect differences in methodology, specificity of diagnosis, route of transmission, infectious dose, or other factors. Reliable diagnosis – determining that all cases diagnosed as SARS are true cases of the disease – has been particularly difficult to establish in this outbreak, as diagnosis is made based on a set of non-specific symptoms and clinical signs that are seen in several other diseases.

Prompt isolation
WHO continues to recommend the earliest possible isolation of all suspect and probable cases of SARS. A short time between onset of symptoms and isolation reduces opportunities for transmission to others. It also reduces the number of contacts requiring active follow-up, and thus helps relieve some of the burden on health services. In addition, prompt hospitalization gives patients the best chance of receiving possibly life-saving care should their condition take a critical course.

Update on cases and countries
As of today, a cumulative total of 6903 probable SARS cases and 495 deaths has been reported from 29 countries. This represents an increase of 186 new cases and 17 deaths compared with yesterday. The new deaths occurred in China (5), Hong Kong SAR (11) and Taiwan (1).

[Russell Scott]

I'm tired of listening to WHO from Hellville. These are the same people who have sat on their hands while devastating health crisis arose in Africa, China, and especially in Iraq, under their food for bribes program. The are another evil, worthless pile of UN kleptocrats with a giant PR department, and an ineffective on-the-ground work force.

[per loin]

Look buddy, if you have an opinion, state it! I'm sick of muddle-mouthers, who sqiggle past words as though danger lurked in their sounding.

[EternalHope]

Well put.

[Future Useless Eater]

SARS Mortality Rates [reflects treatment] Based on World Health Organization daily tables                 (Revised:  May 7 pm)
Area	Recoveries to date	Deaths to date	Recent** Death Rate		Active Cases still in Danger	Projected Future Deaths	Projected Cumulative Mortality
Hong Kong	984	204	19.6%		466	91	17.8%
Singapore	150	27	21.4%		27	6	16.2%
China	1487	219	27.9%		2854	796	22.3%
Canada	93	22	25.0%		31	8	20.5%
elsewhere [28 countries]	171	23	18.0%		145	26	14.5%
World-wide [all 32 countries]	2885	495			3523	927	20.6%
**  ( Deaths in the last 7 days) / ( Deaths + Recoveries in the last 7 days)

Observations - Treatment:
• China continues to improve on the treatment end.
  China could soon have death rates as low as everywhere else... but then again, their 'countryside' deaths seem poised to skyrocket, so... never mind.

Trend - Active Cases Still in Danger [reflects containment]
Date	Hong Kong	Singapore	China	Canada	elsewhere 28 countries	World-wide all 32 countries
Apr 22	874	60	708	61	80	1783
Apr 23	831	58	968	62	86	2005
Apr 24	812	55	1058	58	76	2059
Apr 25	781	50	1209	51	78	2169
Apr 26	774	51	1346	47	86	2304
Apr 27 (est.)	738	45	1415	47	98	2336
Apr 28	709	39	1484	47	108	2387
Apr 29	663	38	1833	40	108	2682
Apr 30	641	38	1969	41	132	2821
May 1	604	33	2117	40	117	2911
May 2	563	32	2246	34	119	2994
May 3	544	31	2375	33	114	3097
May 4 (est.)	532	30	2507	33	125	3227
May 5	520	29	2641	33	135	3358
May 6	495	28	2735	33	132	3423
May 7	466	27	2854	31	145	3523
(includes new daily cases... excludes cases resolved by death or recovery)

Observations - Containment:
• Besides the four hotspots, the 'elsewhere' areas still have a containment problem.
• The most likely next disaster-areas will be Taiwan, Philippines, and Mongolia.

[Future Useless Eater]

SARS - Treatment/Containment tables - updated with Wednesday numbers here

[per loin]

Good work! Have you seen that the time until death in Hong Kong is averaging 36 days, and will that affect your figures?

[Future Useless Eater]

Good work! Have you seen that the time until death in Hong Kong is averaging 36 days, and will that affect your figures?

Thanks. I hadn't seen that figure. Is it 36 days from contact? or 36 days from diagnosis/hospitalization?

I had thought HK was resolving active cases recently in about 22 days vs 15 days for Singapore/Canada.
It also wouldn't surprise me if the continually degrading fatal cases hang on as long or longer(on average) than those that recover enough to be sent home(on average).

Where again is your detailed chart of stats?
Thanks in advance, FLE

[Prince Charles]

Many factors complicate efforts to calculate a case fatality ratio while an outbreak is still evolving. Deaths from SARS typically occur after several weeks of illness. Full recovery may take even longer. While an epidemic is still evolving, only some of the individuals affected by the disease will have died or recovered. Only at the end of an epidemic can an absolute value be calculated, taking into account total deaths, total recoveries and people lost to follow-up. Calculating case fatality as the number of deaths reported divided by the number of cases reported irrespective of the time elapsed since they became ill gives an underestimate of the true case fatality ratio.

I spent an hour this morning unsuccessfully trying to impart this knowledge to a disruptor on another thread. I guess some people are slow learners. ;-)

Regards.

[per loin]

My understanding is that it is 36 days from the time hospitalized. The 22 (now 23) day figure is the average hospitalization time for those who recover. My chart is here.

[Beck_isright]

"WHO now estimates that the case fatality ratio of SARS ranges from 0% to 50%"

Why not just use a dartboard?

[aristeides]

81% of active cases now in China.

[aristeides]

Halt of SARS in Vietnam Could Hold Lessons for Other Nations .

[Dog Gone]

That graph makes it easy to see where the problem areas are. It looks you'll be able to replace Canada or Singapore in a few weeks with Taiwan or some other trouble spot.

[harpseal]

Finally the WHO is coming arround to the truth we have recognized on Free Republic for weeks now. Late last night on one of the bottom of the screen crawlers on one of the cable news networks, most likely FOX, but I can not be sure I saw a report out of Taiwan that SARS had spread out of the hospitals and into the general population for transmission of new cases.

Now it may be that the differneces between the Free Republic numbers and the WHO numbers are not significant but I wil stick with the Free Republic numbers for now becaus eI seriously question the assumptions the WHO uses to claim that our formula about deaths divided by (recoveries + deaths) overstaes the mortality. However, let me state the simple truth. SARS is a disease with a mortality rate that is between 15% and 24% in the general population. The number of people who are coming down with SARS is growing geometrically.

[aristeides]

The number of people who are coming down with SARS is growing geometrically.

I think the spread is only geometric in China. The worldwide spread appears to be geometric because more and more the Chinese cases are dominating the numbers.

[FreepForever]

Thanks to both of you for the splendidly put together charts and graphs. They are very useful. Thanks!!

[FITZ]

The incubation period can vary from one case to another according to the route by which the person was exposed, the dose of virus received, and other factors, including immune status.

It would be interesting to know some other factors such as Hepatitis B and Hepatitis C status and some other diseases. The rate of chronic Hepatitis B is 10-20% in SE Asia ---maybe those people are weakened by their cirrhosis and Hepatitis that they die from SARS and others wouldn't get as sick.

[GOPJ]

Thanks for putting me on your ping list...