1918–1920 flu pandemic The
1918 flu was an unusually severe and deadly
strain of H1N1
swine influenza, which killed from 17 to 50 or more million people worldwide over about a year in 1918 and 1920. It was one of the deadliest
pandemics in human
history. Carnegie Library during the Spanish flu pandemic in 1918, all garbed in uniforms and face masks. The 1918 flu caused an abnormally high number of deaths, possibly due to it provoking a
cytokine storm in the body. (The
H5N1 bird flu, also an Influenza A virus, has a similar effect.) After the
1918 flu infected lung cells, it frequently led to overstimulation of the
immune system via release of immune response-stimulating
cytokines (proteins that transmit signals between cells) into the
lung tissue. This leads to extensive
leukocyte migration towards the lungs, resulting in the destruction of lung cells and secretion of blood and mucus into the alveoli and airways. This makes it difficult for the patient to breathe and can result in suffocation. 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. 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.
1976 swine flu outbreak In 1976, a novel swine influenza A (H1N1) caused severe respiratory illness in 13 soldiers, with one death at
Fort Dix, New Jersey. The virus was detected only from 19 January to 9 February and did not spread beyond Fort Dix. 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.
1977 Russian flu The
1977 Russian flu pandemic 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. Later analysis found that the re-emergent strain had been circulating for approximately one year before it was detected in China and Russia. The virus was included in the 1978–79
influenza vaccine.
2009 A(H1N1) pandemic 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." 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. 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. 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. With similar timely sharing of data for Mexican isolates, by 24 April 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". The outbreak had been predicted a year earlier by noticing the increasing number of
replikins, a type of
peptide, found in the virus. On 11 June 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. On 25 October 2009, U.S. President
Barack Obama officially declared H1N1 a
national emergency. 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 have been impacted by an outbreak. 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. H1N1 may induce other 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 2010 review by Dimitroulis Ioannis et al. The 21 March 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." , 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. The research team of Andrew Miller showed pregnant patients are at increased risk. 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. On 26 April 2011, an H1N1 pandemic preparedness alert was issued by the World Health Organization for the Americas. 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". In May 2013, seventeen people died during an H1N1 outbreak in
Venezuela, and a further 250 were infected. 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, while twenty-one more deaths have been reported across the US. Nine people have been reported dead from an outbreak in several Canadian cities, and Mexico reports outbreaks resulting in at least one death. Spanish health authorities have confirmed 35 H1N1 cases in the Aragon region, 18 of whom are in intensive care. On 17 March 2014, three cases were confirmed with a possible fourth awaiting results occurring at the
Centre for Addiction and Mental Health in
Toronto, Ontario, Canada.
2012 India outbreak With more than 300 infections and over 20 deaths, India's health ministry declared an outbreak "well under control" with "no reason to panic" in April 2012.
2015 India outbreak According to the Indian
Health Ministry, 31,974 cases of swine flu had been reported and 1,895 people had died from an outbreak by mid-March.
2017 Maldives outbreak Maldives reported swine flu in early 2017; 501 people were tested for the disease and 185 (37%) of those tested were positive for the disease. Four of those who tested positive from these 185 died due to this disease. The total number of people who have died due to the disease is unknown. Patient Zero was never identified. 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.
2017 Myanmar outbreak Myanmar reported H1N1 in late July 2017. As of 27 July, there were 30 confirmed cases and six people had died. The
Ministry of Health and Sports of Myanmar sent an official request to
WHO to provide help to control the virus; and also mentioned that the government would be seeking international assistance, including from the
UN, China and the United States.
2017–18 Pakistan outbreak Pakistan reported H1N1 cases mostly arising from the city of
Multan, with deaths resulting from the epidemic reaching 42. There have also been confirmed cases in cities of
Gujranwala and
Lahore.
2019 Malta outbreak 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, with more than 30 cases being treated.
2019 Morocco outbreak In January 2019 an outbreak of H1N1 was recorded in Morocco, with nine confirmed fatalities.
2019 Iran outbreak In November 2019 an outbreak of H1N1 was recorded in Iran, with 56 fatalities and 4,000 people hospitalized.
G4 virus The G4 virus, also known as the "G4 swine flu virus" (G4) and "G4 EA H1N1", is a swine influenza virus
strain discovered in China. The virus is a variant
genotype 4 (G4) Eurasian avian-like (EA) H1N1 virus that mainly affects pigs, but there is some evidence of it infecting people. Michael Ryan, executive director of the
World Health Organization (WHO)
Health Emergencies Program, stated in July 2020 that this strain of influenza virus was not new and had been under
surveillance since 2011. The Chinese CDC said it had implemented an influenza surveillance program in 2010, analyzing more than 400,000 tests annually, to facilitate early identification of influenza. The Chinese Ministry of Agriculture and Rural Affairs rebutted the study, saying that the number of pigs sampled was too small to demonstrate G4 had become the dominant strain and that the media had interpreted the study "in an exaggerated and nonfactual way". They also said the infected workers "did not show flu symptoms and the test sample is not representative of the pig population in China". The US
Centers for Disease Control and Prevention (CDC) said the study suggested that human infection by the G4 virus is more common than it was thought to be. The ECDC stated that "the most important intervention in preparing for the pandemic potential of influenza viruses is the development and use of human vaccines ...". While there have been no reported cases or evidence of the virus outside China as of July 2020, ==Infection in pregnancy==