Influenza A virus subtype H1N1
|Influenza A virus subtype H1N1|
Influenza A virus subtype H1N1
In virology, influenza A virus subtype H1N1 (A/H1N1) is a subtype of Influenza A virus. Well known outbreaks of H1N1 strains in humans include the 2009 swine flu pandemic, the 1977 Russian flu pandemic as well as the 1918 flu pandemic. 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. Hemagglutinin 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.
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, for instance in 2004–2005. Other strains of H1N1 are endemic in pigs (swine influenza) and in birds (avian influenza). Its size is 80 to 120 nm (3.1×10−6 to 4.7×10−6 in) in diameter.
Swine influenza (also known as 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.
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. Properly cooking the meat of an infected animal removes the risk of infection.
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.
1918–1920 flu pandemic
The 1918 flu was an unusually severe and deadly strain of H1N1 avian influenza, a viral infectious disease, that killed some 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.
The 1918 flu caused an abnormally high number of deaths, possibly due to it provoking a cytokine storm in the body. (The current 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 cytokines (a protein that invokes the immune response) 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. Because of a striking similarity in the viral RNA of the two 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. 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 disease then spread very rapidly, with the number of confirmed cases rising to 2,099 by 7 May, despite aggressive measures taken by the Mexican government to curb the spread of the disease. 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. 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 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." As of 30 May 2010[update], 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[when?] 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
2017 Maldives outbreak
Maldives reported swine flu in early 2017;[better source needed] 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.
After widespread rumors about Saudi Arabia going to purchase an entire atoll from Maldives, the Saudi Arabian embassy in Maldives issued a statement dismissing the rumors. However, the trip of the Saudi monarch was going forward until it was cancelled later due to the H1N1 outbreak in Maldives.
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
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
2019 Iran outbreak
In November 2019 an outbreak of H1N1 was recorded in Iran, with 56 fatalities and 4,000 people hospitalized.
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. A 2020 peer-reviewed paper from the Proceedings of the National Academy of Sciences (PNAS) stated that "G4 EA H1N1 viruses possess all the essential hallmarks of being highly adapted to infect humans ... Controlling the prevailing G4 EA H1N1 viruses in pigs and close monitoring of swine working populations should be promptly implemented."
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. Of those, 13 A(H1N1) cases were detected, of which three were of the G4 variant.
The study stated that almost 30,000 swine had been monitored via nasal swabs between 2011 and 2018. While other variants of the virus have appeared and diminished, the study claimed the G4 variant had sharply increased since 2016 to become the predominant strain. 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".
According to the study, between 2016 and 2018, 338 swine production workers in China were tested for antibodies of the virus, and 35 (10.4%) had tested positive. Of those workers between the ages of 18 and 35, 20.5% had tested positive. Among another 230 people screened who did not work in the swine industry, 10 (4.4%) tested positive for antibodies, indicating prior infection. Three cases of infection caused by the G4 virus had been documented as of July 2020, with no confirmed cases of human-to-human transmission.
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. Both the European Centre for Disease Prevention and Control (ECDC) and the US CDC stated that, like all flu viruses with pandemic potential, the variant is a concern that will be monitored. 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 ...". Health officials (including Anthony Fauci) have said that the virus should be monitored, particularly among those in close contact with pigs, but it is not an immediate threat. While there have been no reported cases or evidence of the virus outside China as of July 2020, Smithsonian Magazine reported in July 2020 that scientists agree that the virus should be closely monitored, but because it "so far cannot jump from person to person", it should not be a cause for alarm yet.
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. For this reason the Center for Disease Control and Prevention recommends that those who are pregnant are 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.
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.
Since 1 October 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. However, when 2009 Influenza A (H1N1) virus samples were tested, only 4% (of 853 samples) showed resistance to oseltamivir (again, no samples showed resistance to zanamivir). 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. 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.
- Lim, Boon H.; Mahmood, Tahir A. (23 September 2011). "Influenza A H1N1 2009 (Swine Flu) and Pregnancy". Journal of Obstetrics and Gynaecology. 61 (4): 386–93. doi:10.1007/s13224-011-0055-2. PMC 3295877. PMID 22851818.
- "Influenza Summary Update 20, 2004–2005 Season". FluView: A Weekly Influenza Surveillance Report. Centers for Disease Control and Prevention.
- Jilani TN, Jamil RT, Siddiqui AH (30 November 2020). "H1N1 Influenza". H1N1 Influenza in StatPearls. StatPearls. PMID 30020613.
- Brown, Ian H. (1 October 2001). "The pig as an intermediate host for influenza A viruses between birds and humans". International Congress Series. 1219: 173–178. doi:10.1016/S0531-5131(01)00666-5. ISSN 0531-5131.
- "Humans May Give Swine Flu To Pigs In New Twist To Pandemic". Sciencedaily.com. 10 July 2009. Retrieved 16 October 2010.
- "1918 Pandemic (H1N1 virus) | Pandemic Influenza (Flu) | CDC". www.cdc.gov. 22 March 2020. Retrieved 5 June 2020.
- P. Spreeuwenberg; et al. (1 December 2018). "Reassessing the Global Mortality Burden of the 1918 Influenza Pandemic". American Journal of Epidemiology. 187 (12): 2561–2567. doi:10.1093/aje/kwy191. PMC 7314216. PMID 30202996.
- Kobasa D, Jones SM, Shinya K, et al. (January 2007). "Aberrant innate immune response in lethal infection of macaques with the 1918 influenza virus". Nature. 445 (7125): 319–23. Bibcode:2007Natur.445..319K. doi:10.1038/nature05495. PMID 17230189. S2CID 4431644.
- Kash JC, Tumpey TM, Proll SC, et al. (October 2006). "Genomic analysis of increased host immune and cell death responses induced by 1918 influenza virus". Nature. 443 (7111): 578–81. Bibcode:2006Natur.443..578K. doi:10.1038/nature05181. PMC 2615558. PMID 17006449.
- Cheung CY, Poon LL, Lau AS, et al. (December 2002). "Induction of proinflammatory cytokines in human macrophages by influenza A (H5N1) viruses: a mechanism for the unusual severity of human disease?". Lancet. 360 (9348): 1831–37. doi:10.1016/S0140-6736(02)11772-7. PMID 12480361. S2CID 43488229.
- Palese P (December 2004). "Influenza: old and new threats". Nat. Med. 10 (12 Suppl): S82–87. doi:10.1038/nm1141. PMID 15577936. S2CID 1668689.
- Barry, John M. (2004). The Great Influenza: The Epic Story of the Greatest Plague in History. Viking Penguin. ISBN 978-0-670-89473-4.
- Gaydos JC, Top FH, Hodder RA, Russell PK (January 2006). "Swine influenza a outbreak, Fort Dix, New Jersey, 1976". Emerging Infect. Dis. 12 (1): 23–28. doi:10.3201/eid1201.050965. PMC 3291397. PMID 16494712.
- "Pandemic H1N1 2009 Influenza". CIDRAP. Center for Infectious Disease Research & Policy, University of Minnesota. Retrieved 30 July 2011.
- "1977 Russian Flu Pandemic". GlobalSecurity.org. Retrieved 22 December 2011.
- "Origin of current influenza H1N1 virus". virology blog. 2 March 2009. Retrieved 22 December 2011.
- "New Strain May Edge Out Seasonal Flu Bugs". NPR. 4 May 2009. Retrieved 22 December 2011.
- Nakajima, Katsuhisa; Desselberger, Ulrich; Palese, Peter (July 1978). "Recent human influenza A (H1N1) viruses are closely related genetically to strains isolated in 1950". Nature. 274 (5669): 334–339. Bibcode:1978Natur.274..334N. doi:10.1038/274334a0. PMID 672956. S2CID 4207293.
- Wertheim, Joel. O. (2010). "The Re-Emergence of H1N1 Influenza Virus in 1977: A Cautionary Tale for Estimating Divergence Times Using Biologically Unrealistic Sampling Dates". PLOS ONE. 5 (6): e11184. Bibcode:2010PLoSO...511184W. doi:10.1371/journal.pone.0011184. PMC 2887442. PMID 20567599.
- "Interactive health timeline box 1977: Russian flu scare". CNN. Archived from the original on 22 March 2007.
- "Invasion from the Steppes". Time magazine. 20 February 1978. Archived from the original on 30 September 2007.
- "Pandemic Influenza: Recent Pandemic Flu Scares". Global Security.
- "Russian flu confirmed in Alaska". State of Alaska Epidemiology Bulletin (9). 21 April 1978.
- "Deadly new flu virus in US and Mexico may go pandemic". New Scientist. 26 April 2009. Retrieved 26 April 2009.
- Susan Watts (25 April 2009). "Experts concerned about potential flu pandemic". BBC.
- Centers for Disease Control and Prevention (CDC) (April 2009). "Swine influenza A (H1N1) infection in two children – Southern California, March–April 2009". MMWR Morb. Mortal. Wkly. Rep. 58 (15): 400–02. PMID 19390508.
- "Influenza-like illness in the United States and Mexico". Disease Outbreak News. World Health Organization. 24 April 2009. Retrieved 16 October 2010.
- "Viral gene sequences to assist update diagnostics for swine influenza A(H1N1)" (PDF). World Health Organization. 15 April 2009. Retrieved 31 August 2017.
- Butler, Declan (27 April 2009). "Swine flu outbreak sweeps the globe". Nature: news.2009.408. doi:10.1038/news.2009.408.
- "Influenza cases by a new sub-type: Regional Update". Pan American Health Organization (PAHO). Epidemiological Alerts Vol. 6, No. 15. 29 April 2009. Retrieved 31 August 2017.
- "Influenza A(H1N1) – update 19". Disease Outbreak News. World Health Organization. 7 May 2009. Retrieved 16 October 2010.
- "Efforts To Quickly Develop Swine Flu Vaccine". Science Daily. 4 June 2009. Retrieved 28 December 2016.
One company, Replikins, actually predicted over a year ago that significant outbreaks of the H1N1 flu virus would occur within 6–12 months.
- "H1N1 Pandemic – It's Official". 11 June 2009. Archived from the original on 15 June 2009.
- "Obama declares swine flu a national emergency". The Daily Herald. 2009. Retrieved 26 October 2009.
- "The Arrival of H1N1 Influenza: Legal Considerations and Practical Suggestions for Employers". The National Law Review. Davis Wright Tremaine, LLP. 2 November 2009. Retrieved 3 November 2011.
- Mollura DJ, Asnis DS, Crupi RS, et al. (December 2009). "Imaging Findings in a Fatal Case of Pandemic Swine-Origin Influenza A (H1N1)". AJR Am J Roentgenol. 193 (6): 1500–03. doi:10.2214/AJR.09.3365. PMC 2788497. PMID 19933640.
- Dimitroulis I, Katsaras M, Toumbis (October 2010). "H1N1 infection and embolic events. A multifaceted disease". Pneumon. 29 (3): 7–13.
- "Situation updates – Pandemic (H1N1) 2009". World Health Organization. Retrieved 16 October 2010.
- "Pandemic (H1N1) 2009 – update 103". Disease Outbreak News. World Health Organization. 4 June 2010. Retrieved 16 October 2010.
- "H1N1 Pandemic Flu Hits Pregnant Women Hard". Businessweek.com. 24 May 2010. Retrieved 16 October 2010.
- McAlister VC (October 2009). "H1N1-related SIRS?". CMAJ. 181 (9): 616–17. doi:10.1503/cmaj.109-2028. PMC 2764762. PMID 19858268.
- "WHO Issues H1N1 Pandemic Alert". Recombinomics.com. 26 April 2011.
- Rogall, Gail Moede (8 April 2014). "Sea Otters Can Get the Flu, Too". U.S. Department of the Interior, U.S. Geological Survey. Retrieved 11 April 2014.
- "H1N1 flu outbreak kills 17 in Venezuela: media". Reuters. 27 May 2013.
- "North Texas confirmed 20 flu deaths – Xinhua". English.news.cn. Archived from the original on 12 January 2014.
- "9 deaths caused by H1N1 flu in Alberta". Cbc.ca. Retrieved 10 February 2019.
- "Un muerto en Coahuila por influenza AH1N1". Vanguardia.com.mx.
- "Aumentan a 35 los hospitalizados por gripe A en Aragón". Cadenaser.com. 12 January 2014.
- "Three cases of H1N1 reported at CAMH". Thestar.com. 21 March 2014. Retrieved 10 February 2019.
- "India swine flu 'under control'". BBC News. 12 April 2012. Retrieved 20 July 2020.
- "Swine flu toll inches towards 1,900". The Hindu. 19 March 2015. Archived from the original on 20 March 2015. Retrieved 20 July 2020.
- Murhekar M, Mehendale S (June 2016). "The 2015 influenza A (H1N1) pdm09 outbreak in India". Indian J. Med. Res. 143 (6): 821–823. doi:10.4103/0971-5916.192077. PMC 5094123. PMID 27748308.
- "Makeshift flu clinics swamped as H1N1 cases rise to 82". Maldives Independent.com.
- "Breaking: Swine flu gai Ithuru meehaku maruve, ithuru bayaku positive vejje". Mihaaru.com. 25 March 2017.
- "H1N1 death toll rises to three". Maldives Independent.com.
- "Schools to open after flu outbreak". Maldives Independent.com.
- Ankit Panda. "Will Saudi Arabia Purchase an Entire Atoll From the Maldives?". Thediplomat.com.
- "Maldives dismisses claims Saudi Arabia buying atoll". Arabnews.com. 11 March 2017.
- Lone, Wa; Lewis, Simon (27 July 2017). "Myanmar tracks spread of H1N1 as outbreak claims sixth victim". Reuters. Retrieved 16 August 2019.
- Htet Naing Zaw (27 July 2017). "Myanmar Asks WHO to Help Fight H1N1 Virus". The Irrawaddy. Retrieved 16 August 2019.
- "Toll rises to 42 as 3 more succumb to swine flu". The Nation. 24 January 2018. Retrieved 25 January 2018.
- Caruana, Claire; Xuereb, Matthew (18 January 2019). "Swine flu outbreak at Mater Dei hospital, St Vincent de Paul". Times of Malta. Retrieved 16 August 2019.
- "Swine flu outbreak kills 9 in Morocco". Al Arabiya. AFP. 2 February 2019. Retrieved 16 August 2019.
- "آنفولانزا و راههای درمان آن را بشناسید" [Learn about the flu and how to treat it]. yjc.ir (in Persian). Retrieved 3 December 2019.
- "CDC takes action to prepare against 'G4' swine flu viruses in China with pandemic potential" (Press release). Centers for Disease Control and Prevention. 2 July 2020. Retrieved 3 July 2020.
- Sun H, Xiao Y, Liu J, et al. (29 June 2020). "Prevalent Eurasian avian-like H1N1 swine influenza virus with 2009 pandemic viral genes facilitating human infection". Proceedings of the National Academy of Sciences. 117 (29): 17204–17210. doi:10.1073/pnas.1921186117. ISSN 0027-8424. PMC 7382246. PMID 32601207. Lay summary – BBC News (30 June 2010).
- "Recently publicized swine flu not new, under surveillance since 2011: WHO expert". Xinhuanet. 2 July 2020. Retrieved 2 July 2020.
- "The swine flu virus is not a new virus and does not very spread and is pathogenic to humans and animals [translated from Chinese]". Ministry of Agriculture of the People's Republic of China. 2020. Translation via "Eurasian avian-like A(H1N1) swine influenza viruses" (PDF). European Centre for Disease Prevention and Control. 13 July 2020. Retrieved 20 July 2020.
- Cohen, Jon (29 June 2020). "Swine flu strain with human pandemic potential increasingly found in pigs in China". Science. doi:10.1126/science.abd5761.
- "China says G4 swine flu virus not new; does not infect humans easily". Reuters. 4 July 2020. Retrieved 4 July 2020.
- Ives M (30 June 2020). "Scientists say new strain of swine flu virus is spreading to humans in China". New York Times. Retrieved 20 July 2020.
- "Eurasian avian-like A(H1N1) swine influenza viruses" (PDF). European Centre for Disease Prevention and Control. 13 July 2020. Retrieved 20 July 2020.
- Ries J (1 July 2020). "New swine flu discovered in China: why you don't need to worry too much yet". Healthline. Retrieved 3 July 2020.
- Garcia de Jesus E (2 July 2020). "4 reasons not to worry about that 'new' swine flu in the news". Science News. Retrieved 3 July 2020.
- Machemer T (6 July 2020). "New Swine flu strain with pandemic potential isn't cause for alarm". Smithsonian Magazine. Retrieved 20 July 2020.
- Boon, Lim (23 September 2011). "Influenza A H1N1 2009 (Swine Flu) and Pregnancy". Journal of Obstetrics and Gynaecology of India. 61 (4): 389–93. doi:10.1007/s13224-011-0055-2. PMC 3295877. PMID 22851818.
- "Key Facts about Seasonal Flu Vaccine". Centers for Disease Control and Prevention. 2013. Archived from the original on 30 June 2017. Retrieved 31 January 2014.
- "What You Should Know About Flu Antiviral Drugs". Centers for Disease Control and Prevention. Retrieved 31 January 2014.
- "2008–2009 Influenza Season Week 32 ending 15 August 2009". Centers for Disease Control and Prevention. Retrieved 31 January 2014.
- Saito, S; Minakami, H; Nakai, A; Unno, N; Kubo, T; Yoshimura, Y (August 2013). "Outcomes of infants exposed to oseltamivir or zanamivir in utero during pandemic (H1N1) 2009". American Journal of Obstetrics and Gynecology. 209 (2): 130.e1–39.e1. doi:10.1016/j.ajog.2013.04.007. PMID 23583838.
- "Pandemic OBGYN". Sarasota Memorial Health Care System. Archived from the original on 3 February 2014. Retrieved 31 January 2014.
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- Influenza Research Database Database of influenza genomic sequences and related information.
- Centers For Disease Control and Prevention H1N1 Flu (Swine Flu).
- H1N1 Flu, 2009: Hearings before the Committee on Homeland Security and Governmental Affairs, United States Senate, of the One Hundred Eleventh Congress, First Session: 29 April 2009, Coordinating the Federal Response; 21 September 2009, Protecting Our Community: Field Hearing in Hartford, CT; 21 October 2009, Monitoring the Nation's Response; 17 November 2009, Getting the Vaccine to Where It is Most Needed.