Swine Flu

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It is sometimes called the Great Swine Flu epidemic and sometimes the Great Spanish Flu epidemic, but in either case it was ferocious.  World War I killed twenty-one million people in four years; swine flu did the same in its first four months.  Almost 80 percent of American causalities in the First World War came not from enemy fire, but from flu.  In some units the mortality rate was as high as 80 percent.

This passage is from an interesting book I am currently reading called A Short History of Nearly Everything by Bill Bryson.  An (unfortunately) prescient passage in the book describes a certain flu virus we all became familiar with last summer:

From time to time certain strains of virus return.  A disagreeable Russian virus known as H1N1 caused severe outbreaks over wide areas in 1933, then again in the 1950s, and yet again in the 1970s.  Where it went in the meantime each time is uncertain.  One suggestion is that viruses hide out unnoticed in populations of wild animals before trying their hand at a new generation of humans.  No one can rule out the possibility that the Great Swine Flu epidemic might once again rear its head.

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As I predicted, the H1N1 influenza virus has returned to the U.S. this fall.  FluTracker gives a visual representation of the spread of the disease.  

In response to the spread of H1N1, President Obama declared the H1N1 outbreak a national emergency.  The declaration will  ”allow a hospital to set up a make-shift satellite facility for swine flu patients in a local armory or other suitably spacious location, or at another hospital, to segregate such cases for treatment.”  Without the waiver, “[u]nder federal law, if the patients are sent off site …the hospital could be refused reimbursement for care as a sanction.”

However, the national emergency declaration won’t help increase the speed of production for the H1N1 vaccine. The state of New York had previously declared that all health care workers must be vaccinated for against H1N1.  However, the state recently waved this mandate, not because of a change of opinion but because of vaccine shortages.  The FDA has approved an experimental intravenous use of peramivir against H1N1 in emergency cases.  The FDA approval states that “peramivir can be used when other drugs have failed or when delivery by a route other than intravenous is not expected to be feasible.” 

Should people with flu-like symptoms go to the doctor?  The answer is yes.  However, you may have H1N1 even if your test gives a negative result.  The rapid-test version will only give a positive test result for  11 out of every 100 people who actually have the H1N1 virus (at best).

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The Washington Post reports that most Americans are not very concerned about swine flu.  Should they be worried?  Maps from the New England Journal of Medicine and RhizaLabs detail that swine flu is still a problem.

The CDC reports that “from April 15, 2009 to July 24, 2009, states reported a total of 43,771 confirmed and probable cases of novel influenza A (H1N1) infection. Of these cases reported, 5,011 people were hospitalized and 302 people died.”  However, the CDC does not seem too concerned either.  The CDC has decided to discontinue confirmed and probable case counts on July 24, 2009 (aggregate national reports of hospitalizations and deaths will continue ).

So it is logical for Americans to not be concerned about H1N1?  I believe that public health officials still need to take H1N1 very seriously.  It is very likely that a second H1N1 outbreak will occur this fall and winter during flu season.  During the traditional flu season, H1N1 monitoring must increase.

For the average American, however, I believe there is little reason to worry.  This is not because they will not get H1N1, but instead because there are only a few steps they can take to prevent it.  The CDC recommends: avoiding sick people, washing your hands frequently and covering your face with a tissue when you sneeze.  The first two steps will help prevent you from getting H1N1, but both are logical and most people should already be doing these.  The third step will not help prevent you from getting H1N1, but will prevent the spread of the disease to others if you have it.

The Healthcare Economist’s advice to you is wash your hands, avoid sick people and–until flu season begins–worry about something else other than H1N1.

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Let us assume that our null hypothesis is that when someone is sick, it is not swine flu.  A type I error is a false positive.  That is, we claim that the person has the swine flu, when actually then do not.  A type II error is a false negative.  This means that the person has swine flu, but we erroneously conclude that they do not.

What is the probability that someone who has flu-like symptoms actually has swine flu?  We can calculate this using Bayes Rule:

  • P(H1N1|symptoms) = P(Symptoms|H1N1)*P(symptoms)/P(H1N1)

Let us assume that all individuals with swine flu have symptoms so that P(Symptoms|H1N1)=1.  Let us assume 2% of the population gets any type of flu each year and displays symptoms.  Let us assume only .02% of the population gets H1N1.  So, P(symptoms)=0.02 and P(H1N1)=.0002.  Thus we have:

  • P(H1N1|symptoms) = 1*0.02/0.002=.01. 

This means that if we see a random person with the flu like symptoms, there is only a 1% chance that they actually have the swine flu.  

This may explain why the CDC and WHO ignored early warnings from a Washington-based biosurveillance company concerning a possible flu outbreak.  Although there was an increase in the number of cases of influenza, the probability that it was an outbreak of H1N1 (or any type of outbreak) was low.  Although  probability of a false positive was high, the cost of a false negative is also large.  Ex-post, it is obvious that the CDC and WHO should have acted quicker to fight the spread H1N1.  Ex-ante, these organizations likely receive numerous reports of potential outbreaks and acting on every single one–most of which turn out to be false–would be very costly.  Identifying the optimal time to initial school closings and public health warnings is very difficult and must take into account both the probabilities and the costs of type I and type II errors.

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U.S. School Closures, Online Learning

Map

Quotation

  • “Ill-informed optimism is no better than ill-informed gloom.” - Stephanomics Blog

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Closing public spaces may be a good idea to stop the spread of the flu:

Hong Kong uses technology to fight the spread of disease

  • Ever since the 2003 outbreak of SARS, or severe acute respiratory syndrome, Hong Kong has used infrared scanners to measure the facial temperatures of all arrivals at its airport and at its border crossings with mainland China. Dr. Thomas Tsang, the controller of the Hong Kong government’s Center for Health Protection, said Sunday afternoon at a news conference that any traveler who had passed through a city with laboratory-confirmed cases and who arrived in Hong Kong with a fever and respiratory symptoms would be intercepted by officials and sent to a hospital to await testing. “Until that test is negative, we won’t allow him out,” he said.

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The World Health Organization (WHO) declared Swine Flu to be an international public health emergency.

MÉXICO

La Jornada claims that there swine flu has killed 81 people in México and infected 1324.  The entire country of México has closed schools until May 6th according to La Prensa:

El gobierno federal ordenó la sus­pensión de misas, clases–hasta el 6 de mayo– y todo tipo de eventos abiertos o cerra dos en estadios, teatros, cines, bares y discotecas donde se generen aglomeraciones.

Mexico is also considering shutting down all public transportation.

U.S. and CANADA

According to the New York Times, swine flu has been spreading and risks becoming a pandemic.  In fact, the U.S. has declared a public health emergency.  In the U.S. there have been at least 20 confirmed cases:

SPAIN

After a visit to Mexico, 6 Spaniards have possibly contracted swine flu.

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In Mexico, there has been an outbreak of swine flu which has lead to 20 recorded deaths and likely 40 more.  The newspaper el Universal reports that, the Secretaría de Educación Pública suspended classes in the Distrito Federal and the Estado de México.  This means that in the Mexican capital, 5,201 public schools and 3,965 private schools were cancelled.  

Yet this is not just a Mexican issue.  Swine flu has been found among residents of San Diego and Austin.  Further, after a Canadian citizen contracted swine flu after a visit to Mexico, it was the Canadian government that informed Mexico of the possibility of a swine flu outbreak.

Fortunately, the Mexican government claims it has enough drugs to combat the swine flu outbreak.

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