Influenza Virus Mashup

Via Infobae.com: Este fin de semana se vacunará contra la gripe en todo el país.[Anti-flu vaccinations available in the whole country this weekend] Excerpt, with my translation:

El ministro de Salud, Juan Manzur, confirmó que este fin de semana se llevarán a cabo las Jornadas Nacionales de Vacunación contra la gripe con el objetivo de que “no quede ningún argentino que esté dentro de los grupos de riesgo sin acceder a la vacuna, que es totalmente gratuita”. 

Health Minister Juan Manzur has confirmed that this weekend will be the culmination of the National Vaccination Days against flu; the objective is that “no Argentine in a risk group goes without access to the vaccine, which is totally free.”

“Mañana (por hoy) y el domingo todos los centros de vacunación a lo largo y a lo ancho del país van a estar abiertos y trabajando”, manifestó el funcionario antes de precisar que también se aplicarán dosis en hospitales, centros de salud y se instalarán puestos en lugares públicos como la terminal de ómnibus de Retiro, la Costanera Sur y el Zoológico porteño, entre otros.

“Tomorrow [today] and Sunday all vaccination centres, the length and breadth of the country, will be open and working,” the minister said; he added that vaccinations will also be available in hospitals, health centres, and in public places like the Retiro bus terminal, the Costanera Sur, and the Buenos Aires zoo, among other locations.

Via Xinhua: Flu cases in Singapore near epidemic levels. Excerpt:

Epidemic season has hit Singapore as the number of flu cases surged through the roof, local newspaper The Straits Times reported Thursday. 

Although this has been a traditionally high season for influenza and colds collectively called acute respiratory infection, the number of people with the sniffles has been at epidemic, or near-epidemic levels for the past six weeks. 

A high of 18,420 people sought treatment for the flu at government health polyclinics last week, or 4,000 more patients a week than the norm for this time of the year. 

Many of the flu cases involve the pandemic A/H1N1 strain, which created a health scare worldwide last year, but eventually proved to be milder than originally thought. 

According to the Ministry of Health, 29 percent of patients here with flu-like symptoms last week had this bug.

On May 28, WHO published Pandemic (H1N1) 2009 - update 102. Excerpt:

As of 23 May, worldwide more than 214 countries and overseas territories or communities have reported laboratory confirmed cases of pandemic influenza H1N1 2009, including over 18114 deaths. 

WHO is actively monitoring the progress of the pandemic through frequent consultations with the WHO Regional Offices and Member States and through monitoring of multiple sources of information. 

Situation update: 

The most active areas of pandemic influenza virus transmission currently are in parts of the Caribbean and Southeast Asia, where low level circulation is occurring. Except for localized areas of pandemic influenza activity in parts of Chile, there is little evidence of pandemic influenza activity in the temperate zone of the southern hemisphere. 

Of note, Respiratory Syncitial Virus (RSV) is widely circulating in South America resulting in an increase in respiratory disease activity, complicating somewhat the interpretation of syndromic surveillance data from the area. RSV primarily affects children under the age of 5 years. 

Seasonal influenza A viruses continue to be detected at low to sporadic levels in all regions. Influenza B has been reported in increasing but low numbers in South America, where it only recently appeared, while it is decreasing in Asia. 

In the Caribbean and Central America, Cuba and to a much lesser extent Costa Rica, continue to experience active circulation of pandemic influenza virus. In Cuba, current pandemic influenza activity, which began during late February 2010, remains unchanged since reaching a plateau during mid-April 2010; in addition, over the past month, there have been sporadic detections of seasonal influenza B viruses and also evidence of co-circulation of other respiratory viruses. 

In Costa Rica, sustained low levels of pandemic influenza virus have been co-circulating with other respiratory viruses since the beginning of 2010. Several other countries in the region continue to report sporadic detections of seasonal influenza B viruses and low level co-circulation of other respiratory viruses.

(Sat, 29 May 2010 12:29:00 +0000)

 

 

# 4506

 

 

For those who like their science neat and tidy, devoid of ambiguity, and rock solid . . . the following will prove less than satisfying; a discussion of OAS, or Original Antigenic Sin in this month’s EID Journal  along with some valuable analysis by CIDRAP News

 

While fascinating, there are far more questions than answers at this point.  

 

Original Antigenic Sin is a term coined in 1960 by Thomas Francis, Jr. in the article On the Doctrine of Original Antigenic Sin) that postulates that when the body’s immune system is exposed to and develops an immunological memory to one virus, it may be less able to mount a defense against a subsequent exposure to a second slightly different version of the virus.

Original Antigenic sin has been described in relation to influenza viruses, Dengue Fever, and HIV.  You can find a terrific background piece on OAS from last year by Robert Roos in my blog entitled CIDRAP On Original Antigenic Sin

 

 

In the most recent edition of the EID Journal we get a letter suggesting OAS as an explanation for the age shift to a younger demographic we saw with novel H1N1, along with a separate commentary.

 

Admittedly these are confusing, often technical discussions, cloaked in more than a little uncertainty. Luckily Robert Roos, Editor of CIDRAP News, has once again stepped up with an excellent summary of this difficult concept to help us along.

 

No  . . . you won’t find any definitive answers here, but you will find some intriguing questions.

 

 

Volume 16, Number 6–June 2010

Letter

Original Antigenic Sin and Pandemic (H1N1) 2009

Amesh A. Adalja and D.A. Henderson
Author affiliations: University of Pittsburgh Medical Center, Baltimore, Maryland, USA

 

To the Editor: While pandemic (H1N1) 2009 was in its earliest stages, age distribution data indicated surprisingly few cases among persons >65 years of age. The initial assumption was that few persons >65 years of age had yet to be exposed. However, as more data became available from Mexico, Australia, and the United States, the age distribution pattern persisted (1).

This observation raised the question about whether older persons were protected from infection with an influenza virus A (H1N1) strain acquired many years ago. Indeed, data from the Centers for Disease Control and Prevention showed that approximately two thirds of older persons have evidence of immunity to pandemic (H1N1) 2009 virus. In 1960, Thomas Francis proposed the hypothesis of original antigenic sin, a phenomenon whereby a person who as a child was first exposed to a specific influenza virus A would, throughout life, mount an immune response to the virus of childhood, even when exposed to other antigenically dissimilar influenza viruses. In effect, the original antibody response generated by the immune system against a specific influenza viral strain was hypothesized to have colored all future responses to influenza (2).

(Continue . . . )

 

 

 

Volume 16, Number 6–June 2010

Commentary

The Wages of Original Antigenic Sin

David M. Morens, Donald S. Burke, and Scott B. Halstead

What epidemiologist Thomas Francis, Jr. (1900–1969) was thinking when pondering certain inexplicable serologic data from a 1946 influenza vaccine trial may never be known. Whether in religious reverence for the beauty of science or impish delight fueled by the martini breaks of which he was so fond, Francis coined the term “original antigenic sin” to describe a curious new immunologic phenomenon. Elsewhere in this issue, Adalja and Henderson propose that original antigenic sin has altered the population age–specific incidence of infection and disease caused by influenza A pandemic (H1N1) 2009 virus and that public health responses must account for the disruption (1). What is original antigenic sin, what is its immunologic basis, and into what sort of trouble is it getting us?

(Continue . . . )

 

 

Confused yet?   Robert Roos of CIDRAP may not have the answers, but he does organize and explain the questions exceedingly well.  Highly recommended.

 

 

Age profile of H1N1 cases sparks discussion of ‘original antigenic sin’

Robert Roos News Editor

May 28, 2010 (CIDRAP News) – A letter and commentary published this week in Emerging Infectious Diseases explore the idea that “original antigenic sin”—the hypothesis that the first influenza A virus a person encounters in childhood strongly influences his or her immune responses to all related flu viruses encountered later—may explain the partial protection that older people have against the pandemic H1N1 flu virus.

 

As explained in the letter by Amesh A. Adalja and D. A. Henderson of the University of Pittsburgh Medical Center, original antigenic sin (OAS) holds that a person who is exposed to an influenza A virus in childhood will, in later encounters with related flu viruses, mount an immune response primarily directed at the original virus, even when the newly encountered virus is antigenically different. An implication of the theory is that OAS can trigger an ineffective or less effective immune response when a person is exposed to a virus that’s related to the original one.

 

Because H1N1 viruses circulated continually from 1918 until 1957, most people born before 1957 were exposed to them, Adalja and Henderson write. “According to the theory of original sin, these persons may have partial protection from severe disease from infection with the new influenza virus A (H1N1),” they state.

 

The accompanying commentary, written by three other experts, generally concurs that OAS may account for the protection older people enjoy but says it is unclear whether the phenomenon is a bane or a blessing overall.

 

Meanwhile, another virology expert told CIDRAP News that there is conflicting evidence on OAS and that it may not be necessary to invoke such a complex idea to explain older people’s immunity to the pandemic virus.

(Continue . . .)

(Sat, 29 May 2010 01:15:09 GMT)

Reminder: Please do not post whole articles, just snippets and links. Thanks!!

Colombia

?  Strange epidemic kills twelve indigenous children in Choc? (translated) (Link)

?  H (Link)

News for May 28, 2010 is here.

Thanks to all of the newshounds! Special thanks to the newshound volunteers who translate international stories - thanks for keeping us all informed!

Other useful links:

CDC A(H1N1) Site

WHO A(H1N1) Site

WHO H5N1 human case totals, last updated May 6, 2010
Charts and Graphs on H5N1 from WHO
Google Flu Trends (U.S.)
CDC Weekly Influenza Summary
Map of seasonal influenza in the U.S.
CIDPC (Canada) Weekly FluWatch
European CDC Influenza News
UK RCGP Weekly Data on Communicable and Respiratory Diseases
Flu Wiki Main Page

(Fri, 28 May 2010 14:55:00 +0000)

 

 

# 4605

 

 

The wry hurricane joke in Florida is, if you are going to buy a home, make sure there’s enough leg room in the closets.

 

It is only a half-facetious remark.

 

That’s where a lot of families rode out Hurricane Charley and Andrew.

 

A 40 second clip of Hurricane Conditions.  Imagine this going on for hours.

 

 

As a native Floridian in my sixth decade of life, I’ve been through more than a few hurricanes and tropical storms.   Fortunately, I’ve been spared the experience of being in the core of a major storm. 

 

But that hasn’t diminished my respect for these tropical systems.

 

I was in Pass Christian a few months after Camille crushed the Mississippi coastline with CAT 5 winds and surge, and I helped retrieve my brother’s belongings from a devastated New Orleans just a few weeks after Katrina hit.  

 

I’ve seen the destruction first hand, have done rescue work outside in the fringes of a CAT 1 storm (Hurricane Agnes), have ridden out hurricane force gusts aboard my boat (not recommended!), have evacuated to a motel several times in my life, and have sat hunkered down inside a well constructed home in CAT 1 to CAT 2 conditions.

 

Since I live only about 40 miles from the coast, and reside in what is euphemistically called `manufactured housing’ (actually, a 35’ 5th Wheel RV), I’m preparing now for this year’s hurricane season.

 

And yes, I’ll be evacuating should a Hurricane threaten.

 

Although the tropics don’t usually begin to seriously rock & roll until August, early season storms can occur.  Below are a few notable June Hurricanes, including 1957’s Audrey – a CATEGORY 4 storm.

 

 

So Memorial day weekend isn’t too soon to be preparing.  Hurricane Alma in 1966 struck Florida on June 9th.

 

A word is in order about the Hurricane forecast track issued by the National Hurricane Center in Miami. 

 

Below you’ll find two forecast maps for Hurricane IKE in 2008  – six days apart – with the first showing a hit on Miami and the second a strike on the Texas Coast.

 

On September 4th they had the storm 5 days out from Miami, but instead it went in on the northeastern side of Cuba, crossed Cuba twice, then emerged into the Gulf of Mexico where it intensified and proceeded on to Texas.

 

Did the NHC screw up?

 

A closer look of the first forecast map will show that it struck Cuba well within the `zone of probability’ issued on September 4th.  Miami was the center of the zone, true enough. 

 

But five days out, the NHC said it could hit anywhere from Cuba to Jacksonville.  So no . . . the NHC  did not screw up. 

 

The lesson here is, Hurricane forecasts are pretty reliable 24 hours out, but less so at 48hrs.  The margin of error increases at 3, 4, and 5 days in advance.  Which is why the NHC warns people to pay more attention to the `forecast cone’, than to the centerline.

 

This year, due to better forecasting techniques, the NHC will extend its Hurricane and tropical storm watches and warnings lead time by an additional 12 hours. Warnings will now be issued 36 hours in advance and watches 48 hour.

 

But once again, if you are in the 48 hour `cone’, you need to consider yourself vulnerable.

Since my home would be untenable in any hurricane, I’ve already secured a couple of evacuation locations for myself should a serious storm threaten my area.   I hope to get one or two more.

 

I’ve no desire to load up my car and start driving (along with hundreds of thousands of other evacuees) in search of a non-existent motel room.

 

Yes, there are usually public shelters opened in hurricane areas.  But these should be shelters of last resort, reserved for those who were unable (or unwilling) to make other plans.  

 

Besides, anyone who has stayed in one of these shelters will be quick to tell you that they leave a lot to be desired in the comfort department.   If you can do better, you’d be well advised to do so.

 

I’ve got my 72 hr `bug out bag’ already packed. For a complete description, follow the link to Inside My Bug Out Bag, but here are a few pictures of what I keep at the ready 24/7.

 

 

I’ve also a `packing list’ of other items, and can have my car loaded, and ready to go, in less than a hour.  I keep a serious first aid kit in the trunk at all times, and as I’m packing I’ll be running a fresh backup of my computer onto a USB Flash drive.  

 

My `extra supplies’ will extend my ability to be self sufficient to a week or longer.

 

I also carry (in my wallet) my full medical history, just in case I’m injured or sick, and unable to provide it to emergency personnel.  (See Those Who Forget Their History . . .). 

 

Of course, I also have insurance information, Photo ID’s, some cash, pharmacy and Doctor information, and contact names and phone numbers for those out of the area where I can leave word of my status if need be.

 

 

If you don’t intend to evacuate (and if you are in a safe & secure location, you really shouldn’t), you need to make sure you can fend for yourself (and your family) for at least 72 hours – up to a week or longer .

 

Remember, after a major storm:

• The power may be out for days or longer. 
• Phones may be inoperable (including cell phones). 
• Your water may be off, or contaminated. 
• Streets may be impassible, or dangerous, due to downed wires or tree limbs.
• And emergency crews may have their hands full and be unable to respond quickly . . . assuming you can even call for help.

 

After the storm hits is no time to be scrambling for things like food, water, or first aid supplies.  Things that are relatively easy to do today, may be all but impossible to do later.

Via The Globe and Mail: Low H1N1 vaccination rate alarms health experts. Excerpt:

The announcement that less than one-third of Torontonians received the H1N1 vaccine is taking health experts by surprise and creating alarm over potential system-wide deficiencies with the country’s pandemic planning. 

It’s also raising questions over the lack of solid data on vaccination rates across the country and the accuracy of immunization estimates being used by some cities and provinces. 

On Tuesday, Toronto Public Health released a report that showed 28 per cent of Toronto residents received the H1N1 vaccine. That’s far below national estimates from the Public Health Agency of Canada that indicate nearly half of the country’s population received the shot. And it’s lower than the 40 per cent that Ontario typically immunizes against seasonal flu. 

“I’m surprised that it appears to be significantly lower than that,” said Allison McGeer, director of infection control at Toronto’s Mount Sinai Hospital. 

As policy-makers and health experts evaluate the response to the H1N1 pandemic, Toronto’s relatively low vaccination rate signals serious problems that need to be addressed across the system, said Earl Brown, professor in the biochemistry, microbiology and immunology department at the University of Ottawa. 

Vaccine programs in Toronto and elsewhere were dogged by inconsistent communication from the government over the availability of the vaccine and who should receive it first, as well as problems with vaccine supply and the prominence of anti-vaccine advocates who spread myths about immunization risks, Prof. Brown said. 

“When the experts start equivocating, then the public really had to throw up their hands sometimes,” he said. 

Alberta’s chief medical officer of health said the province immunized about 33 per cent of its population and agreed that missteps were made and that pandemic planning needs major improvements. 

“Our view in Alberta is going to focus on what we can do better next time,” André Corriveau said, citing problems with communication, vaccine delivery and public perception of the risk from the H1N1 flu. 

Although most cities and provinces haven’t released official reports into the H1N1 pandemic, many have given estimates of H1N1 vaccination rates. B.C. said it administered vaccine to about 40 per cent of the general population. Quebec said it immunized nearly 60 per cent, while Saskatchewan has said more than half were vaccinated. A report from Ottawa Public Health estimated that more than half of the city’s residents were immunized.

(Fri, 28 May 2010 12:22:00 +0000)

# 4603

 

 

Today is day six of  National Hurricane Preparedness Week, and the focus today is on preparedness.

 

 

One of the toughest jobs for emergency planners is to get the public to prepare for a disaster while the sun is still shining.  Until a threat looms large, most people simply don’t bother.  

But by that time, it may be too late. 

 

Essential items often disappear from store shelves in advance of a storm, and frankly, it takes time and serious thought to decide what really is important, and what isn’t.

 

Here is some of the information provided by the National Hurricane Center on preparedness.

 

 

“Preventing the loss of life and minimizing the damage to property from hurricanes are responsibilities that are shared by all.”

Throughout this Web site, information has been provided regarding actions that you can take based on specific hurricane hazards. The most important thing that you can do is to be informed and prepared. Disaster prevention includes both being prepared as well as reducing damages (mitigation).

Disaster Prevention should include:

• Developing a Family Plan
• Creating a Disaster Supply Kit
• Having a Place to Go
• Securing your Home
• Having a Pet Plan

One of the most important decisions you will have to make is “Should I Evacuate?”

 

If you are asked to evacuate, you should do so without delay.

But unless you live in a coastal or low-lying area, an area that floods frequently, or in manufactured housing, it is unlikely that emergency managers will ask you to evacuate. That means that it is important for you and your family to HAVE A PLAN that makes you as safe as possible in your home.

 

Disaster prevention includes modifying your home to strengthen it against storms so that you can be as safe as possible. It also includes having the supplies on hand to weather the storm. The suggestions provided here are only guides. You should use common sense in your disaster prevention.

 

• DEVELOP A FAMILY PLAN - Your family’s plan should be based on your vulnerability to the Hurricane Hazards. You should keep a written plan and share your plan with other friends or family.
• CREATE A DISASTER SUPPLY KIT - There are certain items you need to have regardless of where you ride out a hurricane. The disaster supply kit is a useful tool when you evacuate as well as making you as safe as possible in your home.
• SECURE YOUR HOME - There are things that you can do to make your home more secure and able to withstand stronger storms.
• ONLINE VULNERABILITY INFO - There are web sites that can give you information about your communities vulnerability to specific hazards. These include hurricanes as well as other weather related hazards.

 

 

 

Discuss the type of hazards that could affect your family. Know your home’s vulnerability to storm surge, flooding and wind.

Locate a safe room or the safest areas in your home for each hurricane hazard. In certain circumstances the safest areas may not be your home but within your community.

Determine escape routes from your home and places to meet. These should be measured in tens of miles rather than hundreds of miles.

Have an out-of-state friend as a family contact, so all your family members have a single point of contact.

Make a plan now for what to do with your pets if you need to evacuate.

Post emergency telephone numbers by your phones and make sure your children know how and when to call 911.

Check your insurance coverage - flood damage is not usually covered by homeowners insurance.

Stock non-perishable emergency supplies and a Disaster Supply Kit.

Use a NOAA weather radio. Remember to replace its battery every 6 months, as you do with your smoke detectors.

Take First Aid, CPR and disaster preparedness classes.

 

 

 

Water - at least 1 gallon daily per person for 3 to 7 days

Food - at least enough for 3 to 7 days
— non-perishable packaged or canned food / juices
— foods for infants or the elderly
— snack foods
— non-electric can opener
— cooking tools / fuel
— paper plates / plastic utensils

Blankets / Pillows, etc.

Clothing - seasonal / rain gear/ sturdy shoes

First Aid Kit / Medicines / Prescription Drugs

Special Items - for babies and the elderly

Toiletries / Hygiene items / Moisture wipes

Flashlight / Batteries

Radio - Battery operated and NOAA weather radio

Telephones - Fully charged cell phone with extra battery and a traditional (not cordless) telephone set

Cash (with some small bills) and Credit Cards - Banks and ATMs may not be available for extended periods

Keys

Toys, Books and Games

Important documents - in a waterproof container or watertight resealable plastic bag
— insurance, medical records, bank account numbers, Social Security card, etc.

Tools - keep a set with you during the storm

Vehicle fuel tanks filled

Pet care items
— proper identification / immunization records / medications
— ample supply of food and water
— a carrier or cage
— muzzle and leash

 

 

Once a storm appears to be targeting your area, you may only have a few hours to prepare or evacuate.

Some essential hurricane resources to get you started include:

 

http://www.fema.gov/hazard/hurricane/index.shtm

 

http://www.ready.gov/america/beinformed/hurricanes.html

 

 

Later today (or perhaps tomorrow), I’ll be back with my own personal hurricane disaster plan, along with other hurricane information, in A Hurricane Reality Check.

(Fri, 28 May 2010 11:14:00 +0000)

 

 

 

# 4602

 

 

Last year, as many of you will recall, there was a huge debate over the inclusion of adjuvants in the pandemic vaccine – additives that help boost the immune response and lower the amount of antigen needed for each shot.

 

By using adjuvants, the limited supply of antigen could be used to make more flu shots, and immunize more people.

 

Another plus to  using adjuvants is that they can induce a wider range of cross-protection, meaning that as a virus `drifts’ antigenically over time, an adjuvanted vaccine is more likely to remain protective. 

 

Although some adjuvants have been used for years (alum and aluminum salts are incorporated into some diphtheria, tetanus and pertussis vaccines), their use in flu vaccines had been limited.

 

 

Unfortunately that meant we had limited data on their safety and effectiveness, particularly in children and pregnant women.  And that lack of data gave a lot of people pause.

 

So much so, that the decision here in the United States was to go with an unadjuvanted vaccine.  Authorities feared that the incorporation of an adjuvant would deter some people from taking the vaccine.    

 

And in fact, one of the reasons cited for the low uptake of pandemic vaccine in Europe was public concern over the inclusion of adjuvants in the vaccine.

 

Now that millions of doses of adjuvanted vaccine have been administered to people of all ages, we are starting to get research papers on their safety and effectiveness.   

 

Today, we get a head-to-head comparison of GSK’s Pandemrix, containing the adjuvant AS03, verses Baxter’s unadjuvanted Celvapan in British children.

 

Although the adjuvanted Pandemrix vaccine was associated with a higher rate of (usually mild) side effects (fever, injection site soreness), it produced a superior immune response.  

 

Children under the age of three receiving Pandemrix showed an impressive  98.2 % immune response rate, compared to just over 80% with the unadjuvanted Celvapan.

 

For older children, the difference was less pronounced, with 99.1% after 2 doses of Pandemrix verses 95.9% for Celvapan.

 

Excerpts from the BMJ research article follow.  The entire study is available for free.

 

Cite this as: BMJ 2010;340:c2649

Research

Safety and immunogenicity of AS03B adjuvanted split virion versus non-adjuvanted whole virion H1N1 influenza vaccine in UK children aged 6 months-12 years: open label, randomised, parallel group, multicentre study

Claire S Waddington, clinical research fellow, et al.

Abstract

Objectives To compare the safety, reactogenicity, and immunogenicity of an adjuvanted split virion H1N1 vaccine and a non-adjuvanted whole virion vaccine used in the pandemic immunisation programme in the United Kingdom.

 

Design Open label, randomised, parallel group, phase II study.
Setting Five UK centres (Oxford, Southampton, Bristol, Exeter, and London).

 

Participants Children aged 6 months to less than 13 years for whom a parent or guardian had provided written informed consent and who were able to comply with study procedures were eligible. Those with laboratory confirmed pandemic H1N1 influenza or clinically diagnosed disease meriting antiviral treatment, allergy to egg or any other vaccine components, or coagulation defects, or who were severely immunocompromised or had recently received blood products were excluded. Children were grouped by age: 6 months-<3 years (younger group) and 3-<13 years (older group). Recruitment was by media advertising and direct mailing. Recruitment visits were attended by 949 participants, of whom 943 were enrolled and 937 included in the per protocol analysis.

 

<SNIP>

 

Conclusions In this first direct comparison of an AS03B adjuvanted split virion versus whole virion non-adjuvanted H1N1 vaccine, the adjuvanted vaccine, while more reactogenic, was more immunogenic and, importantly, achieved high seroconversion rates in children aged less than 3 years. This indicates the potential for improved immunogenicity of influenza vaccines in this age group.

Via Reuters: Who to blame for flu? Maybe the U.S., study finds. The flu in question is H3N2, not H1N1. Excerpt:

The United States may provide an incubating ground for some flu strains, helping them migrate to warmer climates, U.S. researchers said on Thursday. 

For many years, researchers assumed that flu strains were mostly the product of China and Southeast Asia. 

But a team at the University of Michigan, the Howard Hughes Medical Institute and Florida State University found that not all strains of flu circulating in North America die off at the end of influenza season. 

Some of those appear to head to South America, and some migrate even farther, the reported. That may have happened with the H1N1 swine flu pandemic, they added. 

“We found that although China and Southeast Asia play the largest role in the influenza A migration network, temperate regions — particularly the USA — also make important contributions,” said Trevor Bedford of the University of Michigan, whose study appears in the Public Library of Science journal PLoS Pathogens. 

He and his colleagues tested genetic sequences from seasonal flu viruses collected from patients around the world between 1998 and 2009. They built a sort of family tree, charting the relationships among the viruses. 

The new understanding of flu may require public health officials to change some of their strategies for fighting flu, they said. 

For example, aggressive use of antiviral drugs such as Roche AG’s Tamiflu could promote drug resistance if flu strains never really die out in the United States. 

“We found, for instance, that South America gets almost all of its flu from North America,” Bedford said in a statement. 

“This would suggest that rather than giving South America the same vaccine that the rest of the world gets, you could construct a vaccine preferentially from the strains that were circulating in North America the previous season.”