Influenza A virus subtype H1N1

Influenza (H1N1) virus is the subtype of influenza A virus that was the most common cause of human influenza (flu) in 2009, and is associated with the 1918 outbreak known as the Spanish flu.

It is an orthomyxovirus that contains the glycoproteins haemagglutinin and neuraminidase. For this reason, they are described as H1N1, H1N2 etc. depending on the type of H or N antigens they express with metabolic synergy. Haemagglutinin causes red blood cells to clump together and binds the virus to the infected cell. Neuraminidase is a type of glycoside hydrolase enzyme which helps to move the virus particles through the infected cell and assist in budding from the host cells.[1]

Some strains of H1N1 are endemic in humans and cause a small fraction of all influenza-like illness and a small fraction of all seasonal influenza. H1N1 strains caused a small percentage of all human flu infections in 2004–2005.[2] Other strains of H1N1 are endemic in pigs (swine influenza) and in birds (avian influenza).

In June 2009, the World Health Organization (WHO) declared the new strain of swine-origin H1N1 as a pandemic. This strain is often called swine flu by the public media. This novel virus spread worldwide and had caused about 17,000 deaths by the start of 2010. On August 10, 2010, the World Health Organization declared the H1N1 influenza pandemic over, saying worldwide flu activity had returned to typical seasonal patterns.[3]

Influenza A virus subtype H1N1
Virus classification
(unranked): Virus
Realm: Riboviria
Phylum: Negarnaviricota
Class: Insthoviricetes
Order: Articulavirales
Family: Orthomyxoviridae
Genus: Alphainfluenzavirus
Species: Influenza A virus
Serotype: Influenza A virus subtype H1N1

Swine influenza

Swine influenza (swine flu or pig flu) is a respiratory disease that occurs in pigs that is caused by the Influenza A virus. Influenza viruses that are normally found in swine are known as swine influenza viruses (SIVs). The known SIV strains include influenza C and the subtypes of influenza A known as H1N1, H1N2, H3N1, H3N2 and H2N3. Pigs can also become infected with the H4N6 and H9N2 subtypes.[4]

Swine influenza virus is common throughout pig populations worldwide. Transmission of the virus from pigs to humans is not common and does not always lead to human influenza, often resulting only in the production of antibodies in the blood. If transmission does cause human influenza, it is called zoonotic swine flu or a variant virus. People with regular exposure to pigs are at increased risk of swine flu infection. The meat of an infected animal poses no risk of infection when properly cooked.

Pigs experimentally infected with the strain of swine flu that caused the human pandemic of 2009–10 showed clinical signs of flu within four days, and the virus spread to other uninfected pigs housed with the infected ones.[5]

During the mid-20th century, identification of influenza subtypes became possible, allowing accurate diagnosis of transmission to humans. Since then, only 50 such transmissions have been confirmed. These strains of swine flu rarely pass from human to human. Symptoms of zoonotic swine flu in humans are similar to those of influenza and of influenza-like illness in general, namely chills, fever, sore throat, muscle pains, severe headache, coughing, weakness, and general discomfort. The recommended time of isolation is about five days.

Notable incidents

Spanish flu

The Spanish flu, also known as la grippe, La Gripe Española, or La Pesadilla, was an unusually severe and deadly strain of avian influenza, a viral infectious disease, that killed some 50 to 100 million people worldwide over about a year in 1918 and 1919. It is thought to be one of the deadliest pandemics in human history.

The 1918 flu caused an unusual number of deaths, possibly due to it causing a cytokine storm in the body.[6][7] (The current H5N1 bird flu, also an Influenza A virus, has a similar effect.)[8] The Spanish flu virus infected lung cells, leading to overstimulation of the immune system via release of cytokines into the lung tissue. This leads to extensive leukocyte migration towards the lungs, causing destruction of lung tissue and secretion of liquid into the organ. This makes it difficult for the patient to breathe. In contrast to other pandemics, which mostly kill the old and the very young, the 1918 pandemic killed unusual numbers of young adults, which may have been due to their healthy immune systems mounting a too-strong and damaging response to the infection.[9]

The term "Spanish" flu was coined because Spain was at the time the only European country where the press were printing reports of the outbreak, which had killed thousands in the armies fighting World War I (1914-1918). Other countries suppressed the news in order to protect morale.[10]

Fort Dix outbreak

In 1976, a novel swine influenza A (H1N1) caused severe respiratory illness in 13 soldiers with 1 death at Fort Dix, New Jersey. The virus was detected only from January 19 to February 9 and did not spread beyond Fort Dix.[11] Retrospective serologic testing subsequently demonstrated that up to 230 soldiers had been infected with the novel virus, which was an H1N1 strain. The cause of the outbreak is still unknown and no exposure to pigs was identified.[12]

Russian flu

The 1977–1978 Russian flu epidemic was caused by strain Influenza A/USSR/90/77 (H1N1). It infected mostly children and young adults under 23; because a similar strain was prevalent in 1947–57, most adults had substantial immunity. Because of a striking similarity in the viral RNA of both strains – one which is unlikely to appear in nature due to antigenic drift – it was speculated that the later outbreak was due to a laboratory incident in Russia or Northern China, though this was denied by scientists in those countries.[13][14][15] The virus was included in the 1978–1979 influenza vaccine.[16][17][18][19]

See also 1889–1890 flu pandemic for the earlier Russian flu pandemic caused either by H3N8 or H2N2

2009 A(H1N1) pandemic

AntigenicShift HiRes vector
Illustration of influenza antigenic shift

In the 2009 flu pandemic, the virus isolated from patients in the United States was found to be made up of genetic elements from four different flu viruses – North American swine influenza, North American avian influenza, human influenza, and swine influenza virus typically found in Asia and Europe – "an unusually mongrelised mix of genetic sequences."[20] This new strain appears to be a result of reassortment of human influenza and swine influenza viruses, in all four different strains of subtype H1N1.

Preliminary genetic characterization found that the hemagglutinin (HA) gene was similar to that of swine flu viruses present in U.S. pigs since 1999, but the neuraminidase (NA) and matrix protein (M) genes resembled versions present in European swine flu isolates. The six genes from American swine flu are themselves mixtures of swine flu, bird flu, and human flu viruses.[21] While viruses with this genetic makeup had not previously been found to be circulating in humans or pigs, there is no formal national surveillance system to determine what viruses are circulating in pigs in the U.S.[22]

In April 2009, an outbreak of influenza-like illness (ILI) occurred in Mexico and then in the United States; the CDC reported seven cases of novel A/H1N1 influenza and promptly shared the genetic sequences on the GISAID database.[23][24] With similar timely sharing of data for Mexican isolates,[25] by April 24 it became clear that the outbreak of ILI in Mexico and the confirmed cases of novel influenza A in the southwest US were related and WHO issued a health advisory on the outbreak of "influenza-like illness in the United States and Mexico".[26] The disease then spread very rapidly, with the number of confirmed cases rising to 2,099 by May 7, despite aggressive measures taken by the Mexican government to curb the spread of the disease.[27] The outbreak had been predicted a year earlier by noticing the increasing number of replikins, a type of peptide, found in the virus.[28]

On June 11, 2009, the WHO declared an H1N1 pandemic, moving the alert level to phase 6, marking the first global pandemic since the 1968 Hong Kong flu.[29] On October 25, 2009, U.S. President Barack Obama officially declared H1N1 a national emergency[30] Despite President Obama's concern, a Fairleigh Dickinson University PublicMind poll found in October 2009 that an overwhelming majority of New Jerseyans (74%) were not very worried or not at all worried about contracting the H1N1 flu virus.[31] However, the President's declaration caused many U.S. employers to take actions to help stem the spread of the swine flu and to accommodate employees and / or workflow which may be impacted by an outbreak.[32]

A study conducted in coordination with the University of Michigan Health Service — scheduled for publication in the December 2009 American Journal of Roentgenology — warned that H1N1 flu can cause pulmonary embolism, surmised as a leading cause of death in this pandemic. The study authors suggest physician evaluation via contrast enhanced CT scans for the presence of pulmonary emboli when caring for patients diagnosed with respiratory complications from a "severe" case of the H1N1 flu.[33] However pulmonary embolism is not the only embolic manifestation of H1N1 infection. H1N1 may induce a number of embolic events such as myocardial infarction, bilateral massive DVT, arterial thrombus of infrarenal aorta, thrombosis of right external Iliac vein and common femoral vein or cerebral gas embolism. The type of embolic events caused by H1N1 infection are summarized in a recently published review by Dimitroulis Ioannis et al.[34]

The March 21, 2010 worldwide update, by the U.N.'s World Health Organization (WHO), states that "213 countries and overseas territories/communities have reported laboratory confirmed cases of pandemic influenza H1N1 2009, including at least 16,931 deaths."[35] As of May 30, 2010, worldwide update by World Health Organization(WHO) more than 214 countries and overseas territories or communities have reported laboratory confirmed cases of pandemic influenza H1N1 2009, including over 18,138 deaths.[36] The research team of Andrew Miller MD showed pregnant patients are at increased risk.[37] It has been suggested that pregnant women and certain populations such as native North Americans have a greater likelihood of developing a T helper type 2 response to H1N1 influenza which may be responsible for the systemic inflammatory response syndrome that causes pulmonary edema and death.[38]

On 26 April 2011, an H1N1 pandemic preparedness alert was issued by the World Health Organization for the Americas.[39] In August 2011, according to the U.S. Geological Survey and the CDC, northern sea otters off the coast of Washington state were infected with the same version of the H1N1 flu virus that caused the 2009 pandemic and "may be a newly identified animal host of influenza viruses".[40] In May 2013, seventeen people died during an H1N1 outbreak in Venezuela, and a further 250 were infected.[41] As of early January 2014, Texas health officials have confirmed at least thirty-three H1N1 deaths and widespread outbreak during the 2013/2014 flu season,[42] while twenty-one more deaths have been reported across the US. Nine people have been reported dead from an outbreak in several Canadian cities,[43] and Mexico reports outbreaks resulting in at least one death.[44] Spanish health authorities have confirmed 35 H1N1 cases in the Aragon region, 18 of whom are in intensive care.[45] On March 17, 2014, three cases were confirmed with a possible fourth awaiting results occurring at the Centre for Addiction and Mental Health in Toronto, Ontario, Canada.[46]

2015 India outbreak

Swine flu was reported in India in early 2015. The disease affected more than 31,000 people and claimed over 1,900 lives.[47]

2017 Maldives outbreak

Maldives reported Swine flu in early 2017.[48] 501 people were tested for the disease; 185 (37%) of those tested were positive for the disease. 4 people from these 185 died due to this disease.[49]

The total number of people who have died due to the disease is unknown. Patient zero was never identified.[50]

Schools were closed for a week due to the disease, but were ordered by the Ministry of Education to open after the holidays even though the disease was not fully under control.[51]

After widespread rumors about Saudi Arabia going to purchase an entire atoll from Maldives, Saudi Arabian embassy in Maldives issued a statement against the rumors.[52][53] However the trip of the Saudi monarch was going forward until it was cancelled later due to the H1N1 outbreak in Maldives.[54]

2017 Myanmar outbreak

Myanmar reported H1N1 in late July 2017. As of 27 July, 30 confirmed cases and 6 people had died.[55] The Ministry of Health and Sports of Myanmar sent official request to WHO to provide help to control the virus; and also mentioned that government would be seeking international assistance, including from the UN, China and the United States.[56]

2017–2018 Pakistan outbreak

Pakistan reported H1N1 cases mostly arising from the city of Multan, with deaths resulting from the epidemic reaching 42.[57] There have also been confirmed cases in cities of Gujranwala and Lahore.

2019 Outbreak in Malta

An outbreak of swine flu in the European Union member state was reported in mid-January 2019, with the island's main state hospital overcrowded within a week. The outbreak is ongoing.

2019 Outbreak in Morocco

Many deaths due to H1N1 have been recorded in the month of January 2019 in Morocco. The outbreak is ongoing. As of February 4, 11 deaths have been reported in various regions of Morocco.

In pregnancy

Pregnant women who contract the H1N1 infection are at a greater risk of developing complications because of hormonal changes, physical changes and changes to their immune system to accommodate the growing fetus.[58] For this reason the Center for Disease Control and Prevention recommends that those who are pregnant to get vaccinated to prevent the influenza virus. The vaccination should not be taken by people who have had a severe allergic reaction to the influenza vaccination. Additionally those who are moderately to severely ill, with or without a fever should wait until they recover before taking the vaccination.[59]

Pregnant women who become infected with the influenza are advised to contact their doctor immediately. Influenza can be treated using antiviral medication, which are available by prescription. Oseltamivir (trade name Tamiflu) and zanamivir (Relenza) are two neuraminidase inhibitors (antiviral medications) currently recommended. It has been shown that they are most effective when taken within two days of becoming sick.[60]

Since October 1, 2008, the CDC has tested 1,146 seasonal influenza A (H1N1) viruses for resistance against oseltamivir and zanamivir. It was found that 99.6% of the samples were resistant to oseltamivir while none were resistant to zanamivir. In 853 samples of 2009 Influenza A (H1N1) virus only 4% showed resistance to oseltamivir, while none of 376 samples showed resistance to zanamivir.[61] A study conducted in Japan during the 2009 H1N1 pandemic concluded that infants exposed to either oseltamivir or zanamivir had no short term adverse effects.[62] Both amantadine and rimantadine have been found to be teratogenic and embryotoxic (malformations and toxic effects on the embryo) when given at high doses in animal studies.[63]

Additional images

H1N1 virus particles

This colorized transmission electron micrograph shows H1N1 influenza virus particles. Surface proteins on the virus particles are shown in black

H1N1 virus particles II

This colorized transmission electron micrograph shows H1N1 influenza virus particles.


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External links



1976 swine flu outbreak

In 1976, an outbreak of the swine flu, Influenza A virus subtype H1N1 at Fort Dix, New Jersey caused one death, hospitalized 13, and led to a mass immunization program. After the program began, the vaccine was associated with an increase in reports of Guillain-Barré Syndrome, which can cause paralysis, respiratory arrest, and death. The immunization program was ended after approximately 25% of the population of the United States had been administered the vaccine.

Richard Krause, Director of the National Institute of Allergy and Infectious Diseases from 1975 to 1984, writes that the government response to the swine flu outbreak was considered to be too fast and the response to the AIDS epidemic too slow.

2009 Flu pandemic in North America

The 2009 flu pandemic in North America, part of an epidemic in 2009 of a new strain of influenza A virus subtype H1N1 causing what has been commonly called swine flu, began in Mexico and has since spread worldwide.

2009 flu pandemic in Canada

The 2009 flu pandemic in Canada was part of an epidemic in 2009 of a new strain of influenza A virus subtype H1N1 causing what has been commonly called swine flu. In Canada, roughly 10% of the populace (or 3.5 million) has been infected with the virus, with 428 confirmed deaths (as of 20 February 2017); non-fatal individual cases are for the most part no longer being recorded. About 40% of Canadians have been immunized against H1N1 since a national vaccination campaign began in October, with Canada among the countries in the world leading in the percentage of the population that has been vaccinated. The widespread effect of H1N1 in Canada raised concerns during the months leading to the XXI Olympic Winter Games, which took place in Vancouver on February 2010.

2009 flu pandemic in Hong Kong

The 2009 flu pandemic in Hong Kong started with the city's first reported case of influenza A virus subtype H1N1 infection, commonly called swine flu, on 1 May 2009, in a Mexican national who had travelled to Hong Kong via Shanghai. It was also the first reported case of in Asia. As of 25 November 2009, there have been 32,301 confirmed cases of swine flu in the city.

2009 flu pandemic in Oceania

The 2009 flu pandemic in Oceania, part of an epidemic in 2009 of a new strain of influenza A virus subtype H1N1 causing what has been commonly called swine flu, has (as of 27 June 2009) afflicted at over 22,000 people in Oceania, with 56 confirmed deaths. Almost all of the cases in Oceania have been in Australia, where the majority of cases have resulted from internal community spread of the virus. In addition, the government of New Zealand, where most of the remainder of cases in Oceania have occurred, is on high alert for any people travelling into the country with flu-like symptoms.

2009 flu pandemic in South America

The 2009 flu pandemic in South America was part of a global epidemic in 2009 of a new strain of influenza A virus subtype H1N1, causing what has been commonly called swine flu. As of 9 June 2009, the virus had afflicted at least 2,000 people in South America, with at least 4 confirmed deaths. On 3 May 2009, the first case of the flu in South America was confirmed in a Colombian man who recently travelled from Mexico – since then, it has spread throughout the continent. By far, the most affected country has been Chile, with more than 12,000 confirmed cases, 104 deaths, and the highest per capita incidence in the world.

The World Health Organization warned about the arrival of the winter in the southern hemisphere, where there are seasonal peaks of flu, that could increase the number of infections.

2009 flu pandemic in Turkey

The 2009 flu pandemic was a global outbreak of a new strain of influenza A virus subtype H1N1, first identified in April 2009, termed Pandemic H1N1/09 virus by the World Health Organization (WHO) and colloquially called swine flu. The outbreak was first observed in Mexico, and quickly spread globally. On 11 June 2009, WHO declared the outbreak to be a pandemic. The overwhelming majority of patients experience mild symptoms", but some persons are in higher risk groups, such as those with asthma, diabetes, obesity, heart disease, or who are pregnant or have a weakened immune system. In the rare severe cases, around 3–5 days after symptoms manifest, the sufferer's condition declines quickly, often to the point respiratory failure.

The virus reached Turkey in May 2009. A U.S. citizen, flying from the United States via Amsterdam was found to be suffering from the swine flu after arriving at Istanbul's Atatürk International Airport. Turkey is the 17th country in Europe and the 36th country in the world to report an incident of swine flu.

The Turkish Government has taken measures at the international airports, using thermal imaging cameras to check passengers coming from international destinations.The first case of person to person transmission within Turkey was announced on 26 July 2009.

On 2 November, the Turkish Health Ministry began administering vaccines against H1N1 influenza, starting with health workers.After a slow start, the virus spread rapidly in Turkey and the number of cases reached 12,316. First death confirmed on 24 October and death toll reached 627.

2009 flu pandemic in Ukraine

The 2009 flu pandemic is a global outbreak of a new strain of influenza A virus subtype H1N1, first identified in April 2009, termed Pandemic H1N1/09 virus by the World Health Organization (WHO) and colloquially called swine flu. The outbreak was first observed in Mexico, and quickly spread globally. On the 11th June 2009, the WHO declared the outbreak to be a pandemic. The overwhelming majority of patients experience mild symptoms", but some persons are at higher risk of suffering more serious effects; such as those with asthma, diabetes, obesity, heart disease, or those who are pregnant or have a weakened immune system. In the rare severe cases, around 3–5 days after symptoms manifest, the sufferer's condition declines quickly, often to the point respiratory failure. Although Ukraine was not (very) affected at first there was on outbreak of the virus in Western Ukraine early November 2009 which led to the closing of public buildings and meetings for three weeks.As of December 2009 more than two million people had fallen ill since Ukraine's flu epidemic began and about 500 people of those died of flu and flu-like illnesses and its complications (pneumonia) of the 46 million people living in Ukraine. Ukraine is one of the most affected (8th) by swine flu country's in Europe.According to Ukrainian Justice Minister Mykola Onischuk the epidemiological situation during October–December 2009 hasn't influenced the death rate in Ukraine.Ukraine has two laboratories capable of identifying influenza strains.

2009 in Singapore

The following lists events that happened during 2009 in the Republic of Singapore.

Amurensin K

Amurensin K is an oligostilbene. It is a resveratrol tetramer found in Vitis amurensis. Preliminary tests have shown it to be an effective neuraminidase inhibitor against the influenza A virus subtype H1N1.

BioCryst Pharmaceuticals

BioCryst Pharmaceuticals, Inc. is a pharmaceutical company headquartered in Durham, North Carolina. The company focuses on orphan & autoimmune diseases, and antivirals. The company's most advanced drug candidate is the antiviral Peramivir, which is approved in Japan, Korea and China. It was authorized by the FDA for emergency use in the treatment of patients hospitalized with influenza during the 2009-10 influenza A virus subtype H1N1 pandemic.

Bre Payton

Bre Payton (June 8, 1992 – December 28, 2018) was an American conservative writer. She was a commentator for The Federalist from 2015 to 2018. She was also a conservative commentator on television, appearing on BBC News, Fox News, Fox Business, and the One America News Network. She died of Influenza A virus subtype H1N1 and meningitis on December 28, 2018.


Lymphocytopenia is the condition of having an abnormally low level of lymphocytes in the blood. Lymphocytes are a white blood cell with important functions in the immune system. The opposite is lymphocytosis, which refers to an excessive level of lymphocytes.

Lymphocytopenia may be present as part of a pancytopenia, when the total numbers of all types of blood cells are reduced.

Pandemic H1N1/09 virus

The Pandemic H1N1/09 virus is a swine origin Influenza A virus subtype H1N1 virus strain responsible for the 2009 flu pandemic. For other names see the Nomenclature section below.


Pandemrix is an influenza vaccine for influenza pandemics, such as the H1N1 2009 flu pandemic colloquially called the swine flu. The vaccine was developed by GlaxoSmithKline and patented in September 2006.The vaccine is one of the H1N1 vaccines approved for use by the European Commission in September 2009 upon the recommendations of the European Medicines Agency (EMEA). The vaccine is only approved for use when an H1N1 influenza pandemic has been officially declared by the World Health Organization (WHO) or European Union (EU). This vaccine was initially developed as a pandemic mock-up vaccine using an H5N1 strain.In August 2010, The Swedish Medical Products Agency (MPA) and The Finnish National Institute for Health and Welfare (THL) launched investigations regarding the development of narcolepsy as a possible side effect to Pandemrix flu vaccination in children, and found a minimum 6.6 fold increased risk among children and youths, resulting in a minimum of 3.6 additional cases of narcolepsy per 100,000 vaccinated subjects.


A petechia is a small (1–2 mm) red or purple spot on the skin, caused by a minor bleed from broken capillary blood vessels.Petechia refers to one of the three descriptive types of bleeding into the skin differentiated by size, the other two being purpura and ecchymosis. Petechiae are by definition less than 3 mm.

The term is almost always used in the plural, since a single lesion is seldom noticed or significant.

Primary Mathematics World Contest

The Primary Mathematics World Contest or PMWC takes place every year in Hong Kong. It was first held in 1997, and is sponsored by the Po Leung Kuk organization. The competition has attracted teams from around the world. Each team consists of 4 members, all of whom must be 13 or younger as of September 1, and must not be enrolled in a secondary institution (or the equivalent).

However, the 13th Primary Mathematics World Contest (2009) was canceled due to the WHO declaring the Influenza A virus subtype H1N1 a pandemic. A similar event occurred in the 7th Primary Mathematics World Contest (2003) where the committee postponed the contest to November 2003 due to SARS. The PMWC committee provides room and board at the Panda Hotel in Tsuen Wan.

Russian flu

Russian flu may refer to:

Influenza A virus subtype H2N2#Russian flu, the conjectured cause of the 1889–1890 flu pandemic

Influenza A virus subtype H1N1#Russian flu, the 1977-1978 flu epidemic

"Russian Flu", the 5th episode of the 1st season of the television series Northern Exposure

Russian Flu, a 1937 Swedish comedy film

General topics
Influenza A virus
Pandemics and
Related topics

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