Novel H1N1 (nH1N1) Influenza Situation Report
Marion County Health Department, IN 2009-11-06, 3:00pm
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The characteristics of nH1N1 are roughly similar to those of usual seasonal influenza, except for a greater chance of nH1N1 infection among non-elderly. A cumulative report and prior reports are at http://www.mchd.com/H1N1/H1N1_situational_reports.htm. This report only includes new or especially timely information. An extensive list of flu-related web sites is at http://www.ynhhs.org/emergency/pdfs/Flu_References.pdf.
Spread
There has been a rapid decrease in flu incident from its peak in mid-October. The portion of emergency department (ED) visits that were for symptoms of influenza-like-illness (ILI) was about 10% in mid-October, and is about 4% now, close to the two highest peaks from the last four flu seasons.[1] In each of our laboratory data sources, the percent of influenza tests that detect influenza is now less than half of what it was in mid-October, and the amount of influenza testing has decreased a similar amount.[2] School absences decreased in all 87 schools in the 6 districts for which we have data, and now range between 1% and 6%.[3]
I have not found examples from our past flu seasons or from
nH1N1 patterns from the southern hemisphere where a decline this steep and long
has been followed by a notable increase within the same influenza season. It seems likely that we are past the
worst of this season’s nH1N1 infection rate in Marion County, although influenza
is very unpredictable, and there are still four months left in our flu
season. A summary of nH1N1 patterns
from the southern hemisphere is appended to this report.
During Oct. 25-31, ISDH confirmed nine nH1N1 deaths in Indiana. Since June 1, there have been 19 nH1N1 flu-related deaths in Indiana, three of these being less than 18 years old. The 19 deaths include three Marion County residents; all three were adults. Among 70 Indiana hospitals surveyed, the burden from ILI-related hospitalizations was elevated (more than doubled) in 24%. The central Indiana region had the state’s lowest rate of ED visits for ILI (7.3 per 100 patients), and the state’s second lowest rate of hospitalizations for ILI (4.3 per 100 inpatient beds). Since September 1, 2009, 98% (325/332) of Indiana specimens that were positive for influenza were nH1N1, 87% being from persons ages 0 to 24 years. Only eight were older than 50.[4]
Nationally, from October 18-24, influenza activity was widespread, 100% of all subtyped influenza A viruses were nH1N1, and 19 nH1N1 flu-related pediatric deaths were reported.[5] As of Nov. 6, most of the country had a decrease in the portion of ED visits for ILI, although much of the northeast U.S. had an increase. Indiana was among 5 states that had a greater than 10% decrease in the number of ED visits for ILI from the previous week.[6]
As of November 1, in the northern hemisphere’s temperate zone, influenza transmission continued to intensify. The southern hemisphere’s temperate zone had little influenza activity.[7]
Symptoms, Severity, Groups At Elevated Risk
See http://www.mchd.com/swine_flu/Cumulative_H1N1_Situation_Report.doc for more information. We know of no significant changes in nH1N1 characteristics.
A study of nH1N1 viral shedding among elementary school students and their household contacts after fever onset found that the median duration of positive rRT-PCR detection was 6 days (range 1-13), and the median duration of culture-positivity was 5 days (range 1-7).[8]
Among California’s 1,088 cases of hospitalization or death due to nH1N1 between April 23 and August 11, 2009, the median age was 27 years (range, 0-92 years). 68% (741/1088) had risk factors for seasonal influenza complications. Infants had the highest hospitalization rates. Persons aged 50 years or older had the highest mortality rates once hospitalized.[9]
Antiviral & Drug Resistance
No sustained transmission of oseltamivir-resistant nH1N1 has been identified. WHO reports 42 virus isolates of oseltamivir resistant influenza virus worldwide, none resistant to zanamivir. Over 10,000 isolates of nH1N1 have been tested and found to be sensitive to oseltamivir.[10]
Vaccine Distribution
This week MCHD immunized almost 7000 people between the two mass clinics aimed at the target groups. Another clinic is scheduled for Saturday, November 7. Information about MCHD’s public nH1N1 clinics may be found at http://www.mchd.com/.
More vaccine was distributed to pediatricians, obstetricians, and family practice offices. Many providers primarily serving the target populations began receiving their second shipments of vaccine this week. School vaccination clinics begin next Monday, November 9 and will continue through December. Marion County physicians may request nH1N1 vaccine by clicking the “H1N1 Vaccine Request Form for Healthcare Providers” link at http://www.mchd.com/.
As of November 3,
a total of at least 205,513 IN residents had received the nH1N1 vaccine. Over half of the doses (52%) went to
people age 6 months to 24 years of age, though the distribution varied greatly
depending on each county’s distribution strategy.[11] Over 20,000 vaccines had been
administered in Marion County, although many of these have not yet been added to
the Indiana state tally.[12]
As of November 3, the US government had 31.8 million nH1N1 vaccine doses shipped or available for shipment, which was 9 million more than last week. The CDC is on track to have 10 million doses available next week.[13]
Appendix: H1N1 Incidence
Patterns From Countries in the Temperate Southern Hemisphere
The influenza season in temperate regions of the southern hemisphere is from May or June to September or October. Their experience with H1N1 provides some of the best information we have about what we might expect our incidence patterns to be.
Overall, national aggregate trends indicate that once a steep decline persists for two weeks, it continues, and the wave is past, with no subsequent wave that season. But the national trends showing a smooth rise and fall of a single influenza wave may hide local experiences of “false summits” in the wave, or of minor increases following some steep decrease from a city’s influenza peak. Looking at Australia’s states, several had false summits at week 5, followed by a 2 week trough, followed by the true peak around week 10.
In Marion County, we had a 6 week gradual increase starting in mid-August, followed by about 2 weeks of rapid increase to our one-week peak in mid-October, followed by the last two or three weeks of rapid decrease. I have not found examples from our past flu seasons or from H1N1 patterns from the southern hemisphere where a decline this steep and long has been followed by a notable increase within the same influenza season.
New Zealand
New Zealand saw a peak 6-8 weeks after their rise began (2 weeks slow increase followed by 6 weeks steep increase). Their 3 week peak was followed by 2 weeks of rapid decrease, then 9 weeks of slower decrease with some bumpiness in the 1st 4 weeks of that.[14]
Australia
Several states in Australia experienced a one to two week decline in influenza, followed by a rise to a new, higher peak. The general pattern for these states was a slow 3 to 4 week increase, followed by a rapid increase about 4 weeks long, followed by a two week trough, followed by a rapid 4 week increase rise to a higher peak.[15]
South Africa
South Africa had 3 weeks of slow increase, followeb by 3 weeks of very rapid increase (starting mid-July), followed by a 4 week peak, 3 weeks rapid decline, and 2-4 weeks slower decline to baseline.[16]
Argentina
Argentina’s increase started in early June, peaked at the end of June, declined somewhat steeply to about 1/6th of the peak by the 3rd week of July, then declined slowly to minimal numbers by late Sept.[17]
Chile
Chile saw a rapid 6-7 week rise from their 1st case, with a 3 week peak, and rapid 5 week decline to 4 weeks of few cases then weeks of no confirmed cases.[18]
Appendix: Trends in
Influenza Incidence
Figure 1: Percentage of Emergency Department Visits for Influenza-Like Illness, Marion County Hospitals, 2008-2009 and the Previous four Seasons
Source: ISDH Public Health Emergency Surveillance System. (DR1144)
This week, the percent of emergency departments visits that were for influenza-like illness decreased. It is currently near the two highest peaks seen in the past four flu seasons. Judging from past patterns and from H1N1 patterns in the southern hemisphere’s flu season that recently ended, it seems likely that we are past the H1N1 peak for this season.
Table 1: Emergency Department Visits for Influenza-like Illness by Age, Marion County, Oct 30 to Nov 5, 2009
ILI Visits as % of
All Visits by
Age Category Total number with
ILI
this Age Group
00-04
45
17.2%
05-17
28
15.5%
18-64
50
4.0%
65+
4
1.8%
Grand Total
127
6.6%
Data Source: ISDH Public Health Emergency Surveillance System (MCHD Epidemiology DR1185)
Figure 2: Influenza A Activity by Antigen Detection, Clarian Pathology Laboratory, 2008-2009
Source: Clinical Virology
Laboratory at Clarian Pathology Laboratory. Roughly 55% of the samples were from a
children’s hospital, with the remainder being from general hospitals or
outpatient settings.
Laboratory tests confirm a decrease in influenza A. National surveillance indicates that 99% of influenza cases are nH1N1.
Figure 3: Influenza Activity by Antigen Detection, Clarian Pathology Laboratory, 2008-2009 and the Previous Seasons
Source: Clinical Virology Laboratory at Clarian Pathology Laboratory.
The percent of laboratory tests that was positive for Influenza A has continued to decrease. It is still far higher than usual for this time of year, and a bit above to the maximum from the lowest of last four flu seasons. It appears likely that we are past this season’s peak.