That total number is unknown, unfortunately. Only estimates are available, no complete confirmed counts have been able to be done for the entire pandemic period. The specifics and counts of cases are no longer being tracked by the World Health Organization (WHO) now that the pandemic has been declared over. Influenza cases are monitored, but specific H1N1/09 counts aren't available separately from other influenza reporting any longer.

The latest information about cases and deaths during 2009-2010 in the UK (separate British figures aren't available), is that there were 28,456 confirmed cases and 474deaths. This calculates to a mortality rate of those infected to be 0.02%, which is typical among all statistics. See more on mortality rate below:

Worldwide:

5 March 2010 -- As of 28 February 2010, According to the World Health Organization (WHO), worldwide more than 213 countries and overseas territories or communities have reported laboratory confirmed cases of pandemic influenza H1N1 2009, including at least 16455 deaths.

US:

3 March 2010 -- As of 12 February 2010, According to the US Centers for Disease Control and Prevention (CDC), it is estimated* that since the beginning of the pandemic, the US has had approximately 57 million cases of A-H1N1/09 Pandemic Swine Flu and approximately 11,690 resulting deaths. Based upon this, an estimate of the mortality rate in the US from the pandemic is 0.02% calculated by the 57 million cases and typical percent of deaths.

In comparison, the CDC and World Health Organization (WHO) have estimated that with seasonal flu, "we see over 30 million cases in the United States. We see 200,000 hospitalizations and, on average, 36,000 deaths." (During the entire fall and winter flu season.) Based upon this, the average mortality rate of seasonal flu in the US would be 0.12 %.

*The CDC stopped keeping track of how many cases of H1N1 there were on an ongoing basis, due to incomplete counts in the reports. Most cases that are not complicated or requiring medical care aren't reported to the health departments and, of those, only a small number have had confirmation by laboratory testing. However, deaths from this infection have been being reported better and more have been confirmed, those numbers are more accurate. The CDC has developed a method of estimation of total counts based on the numbers of reported cases and deaths.

The statistics of this mortality rate variation between the seasonal flu and H1N1/09 swine flu, and other data gathered to date during the pandemic, are under study by epidemiologists. The death rate difference may be attributable to the fact that the especially vulnerable demographic group of the elderly (age 65 and older) suffers the majority of the cases and deaths from seasonal flu (because their weakened immune systems are unable to fight it off before their frail bodies must attempt to deal with the symptoms caused by another new strain of virus).

In contrast, the majority of cases of the pandemic swine flu are among the younger and healthier demographic groups, so that, except for the very young and those with underlying medical conditions, most are able to survive the disease. It is not fully understood yet why the elderly do not contract this virus subtype as easily as the typical seasonal virus subtypes, but speculation is that they may have acquired immunity through prior exposure to a similar virus strain sometime in their lives.

A/California/7/2009 (H1N1)v-like virus is the name of the specific strain that is used to create the vaccines for the Pandemic A-H1N1/09 flu. See below for details on this scientific nomenclature.

One of the more commonly used original names for this pandemic flu was Influenza A, Novel H1N1 Virus (formerly 2009 Swine Flu) and was the name that the Centers for Disease Control and Prevention (CDC) coined and used until they conformed with the World Health Organization (WHO) and began calling it H1N1/09 Pandemic swine flu.

It began being called the Pandemic H1N1/09 Virus by WHO in July 2009 and this is the more commonly used name in the US. And many scientific papers are using H1N1pdm (pdm to represent Pandemic).

It is an Influenza Virus type A, H1N1 is the subtype. As mentioned above, the Centers for Disease Control and Prevention and World Health Organization have called it Influenza A, Novel H1N1 Virus. You may also see it written as the "Novel H1N1 Influenza virus", or "Novel Influenza A, H1N1 virus."

What does the H1N1 part mean?:

These Type A viruses have a protein coating that surrounds them, called a capsid. The surface proteins making up the capsid in this virus strain are haemagglutinin and neuraminidase. These surface proteins are the proteins that can be changed when viruses mutate into new forms allowing them to attack the host cells in different ways and to evade the immunological system of the host.

In the naming convention of viruses, the protein classifications become part of the name as in H1N1; H for the haemagglutinin and N for the neuraminidase. There are different subtypes of viruses using the H and N nomenclature, for example the H5N1 subtype is the Avian (Bird) Flu.

Why change from Swine Flu?:

From a July 7, 2009 Press Briefing by World Health Organization's Dr. Fukuda, who said that WHO has decided to call Influenza A, Novel H1N1: Pandemic H1N1/09 Virus (although, as of early August 2009 this name is not yet being consistently used).

"As you know, since the emergence of the pandemic, the name of the virus has been a difficult issue for many reasons. In the past, we have seen how the naming of viruses by location can stigmatize those locations and we have also seen in this and in other episodes where associating the virus with one animal species or another, can really cause both anxiety and then fears about food and in this particular instance, about pork.

So, in recognition of those issues, what WHO, FAO and OIE did, actually some weeks ago, was to get together several of the experts who work in these organizations and with many of the laboratory experts who work with these organizations, and then we had a meeting - a virtual meeting - in which these issues were discussed and one of the things that we wanted to do was make sure that any naming of the virus was scientifically accurate but also would avoid any kind of adverse reactions to the name or to minimize those as much as possible. Based on those discussions, what the experts decided, calling this a pandemic H1N1/09 virus was a good way to distinguish it from the current seasonal H1N1 viruses and to do so, in a way which was scientifically sound, but also would avoid some of the stigma associated with other options."

The name in other countries:

In some countries they still call it Pig Flu, but usually when a virus is called Pig Flu it is in reference to the type of flu that hogs get, not the types that humans get such as the Novel H1N1/09 influenza (2009 Swine Flu). In some places around the world it is also still called the "Mexican Flu", and in France it is la grippe porcine. It is called Schweinegrippe in Germany, Mexican Flu in the Netherlands, SOIV (Swine Origin Influenza Virus) or H1N1-SOIV in Canada, and la epidemia in Mexico. In Turkey it is called "Domuz Gribi" which means "Pink ill". Other common names are Swine influenza, hog flu, pig flu, swine influenza virus (SIV), and H1N1.

Why Swine Flu in the first place?:

The original swine flu is known as influenza type A, H1N1. The new H1N1/09 Pandemic swine flu originated from a pig who had the original/normal pig flu and then this same pig caught the bird flu H5N1. These viruses combined to make a new and bad case of the flu, and this pig gave it to other pigs. Eventually, one of the pigs infected with the flu that was a combination of bird flu and pig flu, then caught the human flu, which "mixed together" in that pig with the other two types of viruses making an even worse case of the flu with all three types of genetic material. This pig spread that new "triple reassortant" virus to more pigs. When pigs with the triple reassortant virus came in close contact with a human, probably a farm worker, and the human caught this bad flu, from then on it has been spreading around the world. The name has stuck and so it is still called the Swine Flu, Swine Influenza, Hog Flu, and Pig Flu in some places to the chagrin of the Pork Producers Association. See the related question below about what caused the pandemic swine flu for more detail on how the reassortant process created the new strain of pandemic flu.

CALIFORNIA = The location the strain was first identified.

7 = The strain identification number.

2009 = The year the strain was identified.

H1N1 = The antigenic characterization of the H and N proteins.

[Antigenic characterization is a method used to describe influenza proteins neuraminidase (N) and hemagglutinnin (H) and how they have changed.]

(See "What Caused the 2009 Swine Flu" in related questions below.)

Pandemic means " A disease prevalent throughout an entire country, continent or whole world." It also means "A disease that is easily transmissible by humans or animals."

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Current situation - World Health Organization raises Pandemic Level to Level 6. From WHO news conference 11 June 2009: "On the basis of available evidence and these expert assessments of the evidence, the scientific criteria for an influenza pandemic have been met. I have therefore decided to raise the level of influenza pandemic alert from Phase 5 to Phase 6.

The world is now at the start of the 2009 influenza pandemic." The transcripts of the Press Conference can be read or videos watched at the WHO site: http://www.who.int/mediacentre/influenzaAH1N1_presstranscript_20090611.pdf It is important to note that this does not indicate an increase in the severity, just that the criteria for Phase 6 regarding the spread of the Novel H1N1 Influenza have been met. This puts in to place additional steps by WHO and Nations' plans for control and prevention in such a widespread situation of pandemic.

The incubation period of influenza is usually two days but can range from one to five days. What are the symptoms of influenza?

Typical influenza disease is characterized by abrupt onset of fever, aching muscles, sore throat, and non-productive cough. Additional symptoms may include runny nose, headache, a burning sensation in the chest, and eye pain and sensitivity to light. Typical influenza disease does not occur in every infected person. Someone who has been previously exposed to similar virus strains (through natural infection or vaccination) is less likely to develop serious clinical illness.

The Food and Drug Administration (FDA) announced the approval of six vaccines on July 18, 2011. These approved trivalent vaccines for the seasonal flu will all contain vaccine for the H1N1/09 "Swine Flu" and two other viruses suggested by CDC for this season (see more below). These approved vaccines are:

1. Afluria (CSL Limited)

2. Fluarix (Glaxo Smith Kline Biologicals)

3. FluLaval (ID Biomedical Corporation)

4. FluMist (MedImmune Vaccines, Inc.)

5. Fluvirin (Novartis Vaccines and Diagnostics Limited)

6. Fluzone, Fluzone High-Dose, Fluzone Intradermal (Sanofi Pasteur, Inc.)

The Fluzone Intradermal made fy Sanofi Pasteur, Inc. is a new inactivated formulation for administration in the layers of the skin (intradermal injection) instead of the intramuscular (IM) injection. Fluzone Intradermal administration uses a microinjection system with a very fine needle. Approved for those aged 18 through 64. It is marketed under the brand names Intanza and IDflu.

The CDC-approved trivalent vaccines for this flu season will protect against the following three virus strains:

1. A/California/7/09 (H1N1)-like virus (Pandemic (H1N1) 2009 influenza virus)

2. A/Perth/16/2009/ (H3N2)-like virus [A/Victoria/210/2009 X-187]

3. B/Brisbane/60/2008-like virus

In Europe:

Re: Pandemrix made by GlaxoSmithKline (GSK) July 21, 2011 according to Reuters:

LONDON (Reuters Health) Jul 21 2011 - European regulators have recommended restricting the use of GlaxoSmithKline's pandemic flu vaccine Pandemrix because of a potential risk of narcolepsy in children or adolescents.

The European Medicines Agency said on Thursday that Pandemrix should only be used in people under 20 years in the absence of seasonal trivalent influenza vaccines, following its link to very rare cases of narcolepsy in young people.

Overall, the vaccine's benefit-risk balance remains positive, the watchdog added.

More than 31 million doses of Pandemrix have been given to people in 47 countries, and GSK said it had been notified of 335 cases of narcolepsy in those vaccinated as of July 6. Two-thirds of the narcolepsy cases were in Finland and Sweden.

Britain's biggest drugmaker said in a statement it had committed to conduct further research into any potential association between Pandemrix and narcolepsy.

Pandemrix was widely used during the 2009-10 outbreak of H1N1 swine flu, although it was not administered in the United States.

Several other drugmakers, including Novartis , Sanofi , CSL and Baxter also made vaccines against H1N1 flu during the pandemic, which was declared over in August last year.

Finnish and Swedish researchers were the first to raise concerns over a possible narcolepsy link to Pandemrix last August after noting cases in children recently given the GSK shot.

One research team earlier this year suggested children given Pandemrix were nine times more likely to suffer from the condition.

Researchers at Finland's National Institute for Health and Welfare said the increase they found in narcolepsy was "most likely" a joint effect of Pandemrix and some other factor or factors.

For the prior flu seasons:

There were four manufacturers' products approved by The Food and Drug Administration (FDA) for the 2009 A-H1N1 Influenza. The drug companies are: CSL, Novartis, Sanofi Pasteur, and MedImmune.GlaxoSmithKline has not yet had approval for their vaccine in the US but it was approved in Europe.

CSL, Novartis and Sanofi Pasteur have produced the vaccines for injection that are made with inactivated viruses, and MedImmune has produced the nasal spray. None of the vaccines approved in the US by FDA contain adjuvants [A substance added to a vaccine to improve the immune response so that less vaccine is needed.]

Note of Interest:

The U.S. is in "very active discussions" about donating some of its supply to countries that need it. CSL plans to donate vaccine to developing nations in Asia and the South Pacific. They are working on this with the World Health Organization to start by providing as many as 100,000 doses. According to the WHO, an initial distribution of the more than 300 million doses, which were donated by other countries to over 90 countries in need, will begin sometime in November.

Vaccine for H1N1/09 is being made for the US by Melbourne-based CSL Ltd., Sanofi-Aventis SA, based in Paris (aka Sanofi-Pasteur), London-based AstraZeneca Plc (MedImmune subsidiary), and Novartis AG based in Basel, Switzerland. GlaxoSmithKline Plc also is in the process of review by the FDA for their vaccine but it is not approved yet. They have gotten approval for one version of vaccine from the European EMEA, however, and is producing it for use in Europe.

Glaxo initially concentrated on development of its vaccine that contained an adjuvant, which is the version approved by the EMEA. Adjuvants are ingredients used in vaccines to boost the effectiveness. The Department of Health and Human Services in the US had decided to only use vaccines without adjuvants. Glaxo is also now doing clinical tests on vaccines without adjuvants for review by the FDA at a later date.

Besides Glaxo's vaccine, EMEA has also approved manufacture of vaccine for European use by Novartis which also contains an adjuvant.

Other countries have contracted with additional manufacturers.