Saturday, June 29, 2013

CID Journal: Estimates Of Human Infection From H3N2v (Jul 2011-Apr 2012)

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Credit CDC

 

 

# 7435

 

 

In my last post (see CID Journal: Lessons From The `First Wave’ Of H3N2v) I recounted the history of the `first wave’ of the H3N2v swine variant virus (July 2011-Apr 2012) as part of a lead up to a series of articles just published in a supplement to the journal Clinical Infectious Diseases.

 

While only 13 human infections were detected during this 9 month time span, it is considered more than probable that a significant number of cases went undetected.

 

We know with practically every infectious disease you can name, that surveillance and testing only picks up a fraction of the total number of cases.  The proverbial `tip of the iceberg’ or in the case of the graphic below, the `top of the pyramid’

 

surveillance

Credit CDC 

 

As an example, last year the CDC was notified of 5,674 cases of West Nile virus disease in people, including 286 deaths. But the actual number of WNV infections (including mild & asymptomatic cases) may have been well over 100,000.

 

Similarly, we don’t actually know how many people contract, or even die from, influenza each year in the United States. With all of these diseases, the number of cases each year must be estimated, based on available surveillance and testing and mathematical modeling.

 

Which brings us to a study that attempts to extrapolate - using models developed during the opening months of the 2009 H1N1 pandemic - how many undetected cases of H3N2v may have occurred during this first wave (July 2011 – April 2012) when only 13 cases were confirmed.

 

Their results – that more than 2,000 human infections from H3N2v may have occurred during this time -  I suspect, will surprise a lot of people.

 

 

Estimates of the Number of Human Infections With Influenza A(H3N2) Variant Virus, United States, August 2011–April 2012

Matthew Biggerstaff, Carrie Reed, Scott Epperson, Michael A. Jhung, Manoj Gambhir, Joseph S. Bresee, Daniel B. Jernigan, David L. Swerdlow, and Lyn Finelli

Background. Thirteen human infections with an influenza A(H3N2) variant (H3N2v) virus containing a combination of gene segments not previously associated with human illness were identified in the United States from August 2011 to April 2012. Because laboratory confirmation of influenza virus infection is only performed for a minority of ill persons and routine clinical tests may not identify H3N2v virus, the count of laboratory-confirmed H3N2v virus infections underestimates the true burden of illness.

 

<SNIP  Methods>

Results. We estimate that the median multiplier for children was 200 (90% range, 115–369) and for adults was 255 (90% range, 152–479) and that 2055 (90% range, 1187–3800) illnesses from H3N2v virus infections may have occurred from August 2011 to April 2012, suggesting that the new virus was more widespread than previously thought.

 

 


Their estimates range from just under 1,200 cases to nearly 4,000 for this `first wave’. 

The `second wave’ began in late June 2012, and ran well into the fall, resulted in more than 300 confirmed cases. This study strongly suggests that those represented but a tiny fraction of the `true’ number of infections last summer.

 

Granted, the surveillance picture during the second wave – once the news broke that scores of people attending county & state fairs had contracted the virus – likely changed from during the first wave.

 

My guess is that these multipliers may need a bit of tweaking for use with the second wave, to account for more robust surveillance and testing that was put into place. 

 

But even so, the number of undetected cases last summer was likely many-fold greater than the 300 confirmed infections turned up by surveillance.

 

This past week, we learned of four new cases (see CDC FluView Update On H3N2v Cases) linked to attendance at a county fair in Indiana.

 

Given the prevalence of the H3N2v virus in swine, and the increased potential for exposure over county & state fair season (running from June-November), it seems likely we’ll be hearing a good deal more about this variant flu virus in the coming months.

 

Despite these numbers, and apparent limited human-to-human transmission of this virus, this strain has not yet managed to spread efficiently in the community.

 

The CDC maintains an H3N2v and You FAQ page, and offers the following advice for fairgoers and exhibitors.

 

Preventive Actions

CDC Recommendations For People At High Risk:

  • If you are at high risk of serious flu complications and are going to a fair where pigs will be present, avoid pigs and swine barns at the fair this year. This includes children younger than 5 years, people 65 years and older, pregnant women, and people with certain long-term health conditions (like asthma, diabetes, heart disease, weakened immune systems, and neurological or neurodevelopmental conditions).

If you are not at high risk, take these precautions:

  • Don’t take food or drink into pig areas; don’t eat, drink or put anything in your mouth in pig areas.
  • Don’t take toys, pacifiers, cups, baby bottles, strollers, or similar items into pig areas.
  • Wash your hands often with soap and running water before and after exposure to pigs. If soap and water are not available, use an alcohol-based hand rub.
  • Avoid close contact with pigs that look or act ill.
  • Take protective measures if you must come in contact with pigs that are known or suspected to be sick. This includes minimizing contact with pigs and wearing personal protective equipment like protective clothing, gloves and masks that cover your mouth and nose when contact is required.
  • To further reduce the risk of infection, minimize contact with pigs and swine barns.

 

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