Pandemic H1N1 ‘Swine flu’ FACTFILE Pandemic H1N1 ‘Swine flu’ • A novel influenza A H1N1 virus emerged from Mexico in April 2009 and spread so rapidly that the World Health Organization declared a global flu pandemic on 11 June. • Symptoms can include fever, fatigue, lack of appetite, coughing and sore throat and, in about one quarter of cases, vomiting and diarrhoea. • There were two peaks of infection in the UK, the first during the summer months and the second smaller peak over the traditional winter influenza season. In the post-pandemic period the virus reappeared, behaving as a seasonal strain. Influenza overview quickly through the population development of a novel adapted Influenza viruses in human across the globe irrespective of the strain. As the human population may populations are usually classified season. not have encountered the virus before, as seasonal or pandemic. Seasonal Typically, influenza viruses it has little or no immunity (antibodies) influenza usually causes yearly predominately circulate in birds; rarely to the novel strain. Such processes epidemics in the old, the very young they can infect and genetically adapt probably allowed the pandemics in and those with underlying medical to replicate well in other animal hosts. 1918, 1957 and 1968 to occur. Each conditions, and is often associated with Additionally, different viral strains can pandemic was caused by a different cold periods in temperate regions. An infect a single host and exchange that subtype of influenza, named for the influenza virus becomes classified as adapted genetic material, a process outer spike proteins of the virus (H1N1, pandemic when a novel strain emerges made easier as the genome is separated H2N2 and H3N2, respectively), which from an animal reservoir and is not only into eight segments, allowing mixing are the sections of the virus most able to infect humans but also spreads (reassortment) to occur and lead to recognised by the host antibodies. 2 Factfile: H1N1 | www.microbiologysociety.org Coloured transmission electron micrograph of H1N1 swine flu virus particles from the April 2009 outbreak, which originated in Mexico City, Mexico. The 2009 H1N1 strain spread rapidly between them, and thus illness and/or death; however, as the The WHO named the new 2009 virus cause the 2009 pandemic. Since 2009, virus spread the number of severe influenza A(H1N1)pdm09, but it was this virus has repeatedly transmitted cases in the infected population termed ‘swine flu’ in the media as the back into pigs from humans. Other decreased. Unlike H5N1 (‘bird flu’ in virus displayed genetic similarities with reassorted viruses have transmitted the media) and other highly pathogenic existing pig viruses. This influenza virus from pigs to humans, but they have not influenza viruses, the pandemic H1N1 was the result of the reassortment of been able to transmit between humans strain does not normally spread outside four different bird (avian) viruses that as well as the A(H1N1)pdm09 virus. the respiratory tract; this is usually a had adapted to use pigs (and a human trait associated with a milder seasonal intermediary) as hosts and mixing Who is affected? disease. While there were severe and pots for several years, hence the name When the virus was originally detected, fatal cases requiring hospitalisation, ‘swine flu’. The new A(H1N1)pdm09 virus the news from Mexico was alarming – most cases of A(H1N1)pdm09 flu in the was able to transmit to humans and many of the cases resulted in critical UK were mild. Factfile: H1N1 | www.microbiologysociety.org 3 Treating illness (Relenza™) and peramivir. These antivirals work by blocking the release of the virus from infected cells by inhibiting a viral enzyme. Influenza strains that were resistant to oseltamivir were first identified several years ago. Oseltamivir-resistant A(H1N1)pdm09 influenza had been reported in <1% of cases in the last 6 years, according to Public Health England. Oseltamivir resistance most often occurs in patients who are immunosuppressed, who tend to clear the virus more slowly from the body. Pharmacist holding packets of TamifluTM (oseltamivir) capsules. Other drugs are being The normal recommended treatment inside human cells or blocking the developed; some such as favipiravir for flu for healthy individuals is bed spread of virus from one infected cell to target the viral replication machinery rest and a high fluid intake. Remedies the next. whereas others specifically seek to containing paracetamol or ibuprofen In certain situations (as inhibit the virus by temporarily will help ease symptoms such as a happened early in the pandemic), blocking specific host proteins that high temperature and muscle pain. antivirals can be given to prevent flu the virus needs to replicate or by Asprin is not recommended for in healthy people who are exposed to manipulating specific aspects of children. Antibiotics are NOT effective the virus (prophylactic treatment). The the host’s immune system. Neither against viruses such as influenza A(H1N1)pdm09 strain has shown to favipiravir nor peramivir are, as yet, and should only be prescribed to be sensitive to antiviral medications licensed for influenza treatment in treat confirmed secondary bacterial oseltamivir (Tamiflu™), zanamivir the UK. infections, which can lead to bacterial pneumonia. Oseltamivir (Tamiflu) Zanamivir (Relenza) Patients who are deemed to be at Adults, pregnant high risk (e.g. pregnant women, those Adults, pregnant women Safe for use in: women, children and with pre-existing medical conditions and children over 5 babies and young children) from pandemic or seasonal flu strains are prescribed Method of administration: Oral Inhaled antiviral medication. This must be taken quickly to be effective, ideally within Formulation: Capsule or liquid Dry powder 12–48 hours of onset of symptoms. One 75 mg capsule Two 5 mg doses of powder Adult dosage: Antiviral medications do not cure per day for five days per day for five days illness but they can reduce the length of symptoms by about a day and usually Child dosage: Weight-related Same as adult dosage lessen their severity. Antivirals work Common side effects: Nausea and vomiting None by blocking the virus from replicating 4 Factfile: H1N1 | www.microbiologysociety.org As observed in previous pandemics, Phase Containment phase Treatment phase the majority of the cases were in young people under 25, although a reasonable Aim Limit spread of infection Minimise impact of pandemic proportion of middle-aged people were also affected; this is in contrast Suspected cases tested to Suspected cases no longer tested to seasonal flu that predominantly confirm H1N1 infection to confirm H1N1 infection affects the elderly, who have a less Antiviral medication given to Fast access to antivirals through robust immune system. However, when confirmed cases National Pandemic Flu Service they are infected, some groups have Action an increased risk of suffering from taken Schools closed (where medical H1N1 vaccine production complications from an A(H1N1)pdm09 advice deemed it appropriate) fast-tracked infection. These include the elderly, people with long-term health conditions Prophylactic treatment of close Licensed H1N1 vaccine offered including asthma, diabetes and obesity, contacts of infected people with to vulnerable groups in order of women in the later stages of pregnancy antiviral medication priority and those with weakened immune systems. with peak seasonal flu season at with vaccine manufacturers were in the end of December/beginning of place to quickly develop a vaccine The spread of disease January; however, the number of new against a new strain of flu. Vaccines, H1N1 infection spread rapidly in June H1N1 cases did not reach the figures even when fast-tracked, take time to and July 2009 as the majority of people that were expected. The following make. The first vaccine doses in the UK were naïve to the virus and therefore influenza season, 2010–2011, saw a were not ready until 21 October 2009; had no antibodies to it. The number of sustained number of A(H1N1)pdm09 so, when the A(H1N1)pdm09 strain new cases each week started to double infections in the young and middle- first emerged, public health authorities and peaked at 100,000 per week in aged, and a higher-than-expected focused their efforts on containing the England. In October, the number of new level of hospitalisation and critical spread of infection. As more and more cases per week started to increase care patients. The seasonal H1N1 and people became infected, effort was again and a second, smaller peak of H3N2 viruses were supplanted by the centred upon treating the disease to infection occurred. It was predicted newly emerged virus. However, since minimise the impact of the pandemic. that the second wave would coincide 2010–2011, both A(H1N1)pdm09 and A/ The UK officially moved from a H3N2 have circulated as the seasonal containment phase to a treatment phase strains, with A(H1N1)pdm09 dominating on 2 July 2009. the first season, then H3N2 the following To ease the mounting pressure on two winters; currently each virus GPs, the National Pandemic Flu Service predominates on alternate flu seasons. was launched in the UK at the end of Influenza B viruses have co-circulated July. This service provided an online throughout. or telephone diagnosis of a patient’s symptoms and gave them quick access Dealing with the pandemic to antiviral medication if necessary. Most countries, including the UK, Patients were advised to nominate a ‘flu had flu preparedness plans in place. friend’ to collect the medication for them In Britain, before a pandemic alert was to further limit the spread of infection. issued by the WHO, preparations were Much was learnt from the 2009 already underway. Enough antiviral pandemic; the preparedness plans have medication was stockpiled to treat half been modified for each stage of the Illustration of a global influenza pandemic.