In response to WHO daily briefing on H1N1 of Mexico and Worldwide cases, May 3, 2009 update
On Mexican cases
1. case-fatality: it stands at 3.8%, is slightly higher than 1918 swine flu (2.5%) but much lower than 2003 SARS (15%) and 2004 Bird flu (60%).
2. Those who were less than 29 yrs old accounted
for 2/3 of the cases. This is similar to H5N1 that infected mostly
younger population, but different
from SARs that affected mostly senior group (>50%).
3. Confirmed deaths as of 4/28: the fatality or potentiality of the
outbreak is showing an overall trend of declining, at least it's under
control in Mexico.
If the flu outbreak is escalating we will expect persistently
increased, or exponential deaths after first two weeks of first case
reported. But
it's not the case in this Mexican data.
On Worldwide Cases, May 3, 2009
4. Data need: We need for more specific data reporting to make more
sensible analysis and inferences. We need "death cases by age group or
gender" to identify vulnerable groups
by demographic characteristics. For worldwide cases reported by WHO,
we need the same info as discussed above, and particularly, for new
cases (incidence data)
we need information about where they are with or without prior
Mexican contact (which accounts for about 1/3 worldwide cases now).
5. To avoid the hysteria around the world "newly confirmed H1N1
cases".
For those 2/3 cases did not have Mexican contact (and newly confirmed
cases recently and hereafter), we need to know whether these cases are
new
community clusters (i.e., occurred due to sustained person-to-person
contact) or are newly confirmed in the lab due to backlog.
My assumption is that the recently observed increased new cases were,
and may not be totally new cases, but those that would be otherwise
diagnosed as either seasonal or regular flu, had the current strain of
h1n1 not isolated last month. In other words, what we are seeing the
worldwide "rise" or "JUMP" in cases may not be actually due to the
"SPREADING" of the flu but it may be just a confirmation of those
existing infections around the globe. We are just confirming the
infection which will help us focus our intervention. The fact
that cases/deaths in Mexico is not exponentially escalating render
potential evidence to support this speculation.
This is an important message to get across because this assumption, if substantiated, could calm the hysteria around the world that the disease may not be spreading fast in their region, but that we are confirming cases that were not previously confirmed as caused by this particular H1N1 strain, and that we are dealing with a new flu that have similar risk to, if not lower than, the seasonal flu.
Q: Isn't it true that in any normal year its the very young and the very old who die more frequently from flu?
A: Based on the experiences from SARs and H5N1
(the Bird Flu): older people are most vulnerable to SARS, but both older
and younger are NOT
most vulnerable to the bird flu – Younger age group is.
The above statement is only True for the elderly in SARs outbreak. In terms of Case Fatality for SARS, CF:>50% for the seniors aged 65 or older,
According to 2004-2006 data when we had the
largest worldwide cases, it's NOT true for the very young in both SARs
(for age <24, CF<1%) and H5N1
bird flu (for age <10, CF=42% versus age 10-19, CF=73% and 20-29,
CF=62%, versus older group aged >50, CF=18%)
One thing in point is that if the H1N1 (swine
flu) continues to hit younger people the hardest - as we saw in bird flu
cases, we may be able to draw some
parallels from the H5N1 outbreak. Including its temporal distribution,
such as its seasonality, potential re-emerging time points and the
temporal characteristic of patients from admission to death.
Q: Do we have enough confirmed deaths to determine any pattern on age of greatest susceptibility?
A: Not at this point. We may need another week
until the H1N1 outbreak runs the full course of its incubation and
infectivity period (some suggest between 3 days and a week).
Sometime in May we should have enough data to make some
meaningful inferences from worldwide distribution of the disease,
including susceptibility or vulnerability by age over time.
Q: Shouldn't we be seeing an increase in the total numbers in Mexico, with
so many countries now reporting cases?
A: Yes. Based on the analysis of the SARS data of 4/2003- 6/2003 in China
(see figure 1) and our analysis of worldwide H5N1 data (see figure 2, for
2.5 yrs 1/2004-05/2006), if the trends hold true for H1N1, we will see
substantial increase in H1N1 confirmed cases worldwide, including from
Mexico, in next week through early May. However, again if the trends hold
true, we can also reasonably expect the cases will go down dramatically
beginning the week after (i.e., week of 5/3). Note that the rise in cases
in next week is normal and expected - could be due to the incubation
period of H1N1 averaged 1-3 days, and a more accurate and rapid case
identification. Next week will be a critical period to closely watch for
case increase.
Q:
Should the alert level be raised by WHO? Should it be raised by CDC?A: No. Not at this point. There are still many unknowns about the disease particularly in terms of route of transmission (except for Mexican contracts) and H1N1’s sustained ability to pass among human. We do not see a clear pattern of other route of transmission, and based on the passage of the past 72 hrs since next week (i.e., incubation period that could likely result in high yield cases), the H1N1 virus, taking swine, birds, pigs as reservoirs, did not seem in a urgency to mutate, recombine, or replicate in human beings to survive.
Q: What does the data show about the effectiveness of masks, such as the ones being handed out in Mexico?
A: The masks could be helpful in avoiding the spread of sneeze and facial contact in exchange greetings, but may have limited utility in avoiding the spread of the airborne H1N1 virus.
Q: What was learned from the SARS experience about what works and what does
not work in regard to responding to an outbreak?
A: I think that the world has learned a great deal from previous SARS/H5N1
outbreaks, including heightened inspection and quarantined on air travel
which is believed to be major route of transmission so far. Consideration
could be give to extend this line of defense to air travelers’ origin, not
only destination. This could prevent the virus spread through air travel
at the first place.
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References:
Presented in APHA -134th American Public Health Association Annual Meeting & Exposition (November 4-8, 2006) in Boston, MA. http://apha.confex.com/apha/134am/techprogram/paper_144102.htm
