4 Methodology
4.4 Hygroscopic properties
Influenza A is ‘one of the most efficient and infectious pathogens known’41 and causes a respiratory disease which is rapidly and readily spread between people. The ability of a person to infect a number of others in an unimmunised population is known as the replication number (or Ro). Where the Ro is less than 1 a disease is likely to disappear.42 The potential for widespread disease is likely once the Ro is at 1.5-2,43 but this may be a crude assessment for a number of reasons.44 Because the influenza virus replicates quickly, the disease can spread rapidly even with a comparatively low Ro. It has been estimated that the Ro for pandemic influenza in 1918 was between 1.4 and 2.8, while in 2009 the Ro was 1.4 to 1.6.45 However, as the Ro is usually measured by looking at large population aggregates it may not hold true for smaller groups including ‘military camps and ships’ where the Ro can be higher.46
The typical incubation time, that is time from influenza exposure to evidence of illness, for influenza is between 1-4 days (with an average of 2 days). It is known that a person sheds most virus (ie, is most infective) during the early course of their illness when fever is present; and, the more severe the symptoms, the greater the risk to
40 Van-Tam and Sellwood, above n 19, 20.
41 Clinical Aspects of Influenza (Allen W. Kirchner) in Ryan, above n 18, 73.
42 https://practice.sph.umich.edu/micphp/epicentral/basic_reproduc_rate.php last accessed 16 February 2016.
43 The Ro for diseases, such as pertussis and measles, is over 12 meaning they are highly infectious:
https://practice.sph.umich.edu/micphp/epicentral/basic_reproduc_rate.php last accessed 16 February 2016.
44 Fisman, Leung and Lipsitch, above n 13.
45 http://www.healthline.com/health/r-nought-reproduction-number#TheBasics2 last accessed 7 March 2016, see also summary in Maria D Van Kerkhove et al, 'Epidemiologic and virologic assessment of the 2009 influenza A (H1N1) pandemic on selected temperate countries in the Southern Hemisphere: Argentina, Australia, Chile, New Zealand and South Africa' (2011) 5(6) Influenza and Other Respiratory Viruses e487, e492.
46 Christophe Fraser et al, 'Influenza transmission in households during the 1918 pandemic' (2011) 174(5) American Journal of Epidemiology 505, 506.
35 others.47 Children, and those who are immunocompromised,48 are likely to be infective for longer periods of time.49 Perhaps of greater concern is the fact that asymptomatic, but infected persons, may contribute to the spread of influenza.50 Some researchers believe asymptomatic transmission is unlikely51 while others believe that as many as 1 in 3 infected persons may be asymptomatic yet contributing to disease spread.52
Although influenza can be spread in a number of ways, it is believed that transmission occurs primarily by way of droplet dispersal and direct contact.
However, despite having first been identified in 193353 with virology ‘truly coming of age’ by 195054 and with allegedly more known about the genetic structure of the
‘influenza type A virus than any other human virus’,55 uncertainty remains about the relative importance of various means of transmission.56 For example, although it is estimated that the influenza virus can survive on surfaces for an estimated 1-2 days57 there is currently no evidence that ‘contact with contaminated surfaces results in transmission of infection’.58
47 Van-Tam and Sellwood, above n 19, 58.
48 An immunocompromised person has a less responsive immune system meaning that they are more prone to infection: http://infectiousdiseases.about.com/od/glossary/g/immunocompromised.htm last accessed 21 January 2015.
49 Influenza Transmission and Infection Control Issues (Joanne Enstone and Ben Killingley) in Van-Tam and Sellwood, above n 19 68.
50 Robert A Weinstein et al, 'Transmission of Influenza: Implications for Control in Health Care Settings' (2003) 37(8) Clinical Infectious Diseases 1094, 1095.
51 Elini Patrozou and Leonard A Mertmel, 'Does Influenza Transmission Occur from Asymptomatic Infection or Prior to Symptom Onset?' (2009) 124(2) Public Health Reports 193. The authors conclude (196) there was ‘limited evidence to suggest the importance of [asymptomatic]
transmission’.
52 Fabrice Carrat et al, 'Time lines of infection and disease in human influenza: a review of volunteer challenge studies' (2008) 167(7) American Journal of Epidemiology, 775.
53 Smith, Andrewes and Laidlaw, above n 1.
54 Jeffery K Taubenberger, Johan V Hultin and David M Morens, 'Discovery and characterization of the 1918 pandemic influenza virus in historical context' (2007) 12(4 Pt B) Antiviral Therapy, 581, especially 586-7.
55 Ryan, above n 18, 62.
56 Enstone and Killingley in Van-Tam and Sellwood, above n 19, 69-70.
57 Axel Kramer, Ingeborg Schwebke and Günter Kampf, 'How long do nosocomial pathogens persist on inanimate surfaces? A systematic review' (2006) 6(1) BMC Infectious Diseases 130.
58 Enstone and Killingley in Van-Tam and Sellwood, above n 19, 69.
36 The influenza virus gains entry to the human body by contact with mucosal tissues (this includes eyes, mouth and nose) as well as the respiratory tract.59 At rest a healthy adult takes around 12–20 breaths a minute, exchanging approximately 500 mls of air each time — at a conservative 12 breaths x 60 (per hour) x 0.5 litre that means a person breathes in (and exhales) around 360 litres of air an hour. This normal breathing is both a means of spread as well as a way of inhaling the virus. An ill person, especially if they have a fever, is likely to have a higher respiratory rate. While you cannot stop breathing, it is believed that good cough etiquette (i.e. covering the mouth when coughing or sneezing) as well as maintaining a distance of around 3 feet from a person who is coughing or sneezing might reduce the risk of disease transfer.60 Signs and symptoms of influenza typically come on abruptly and can include a high fever61 (of 3-4 days duration), headache, a history of fatigue/weakness (which may result in severe exhaustion) accompanied by severe aches and pains. A cough and chest discomfort is common, however an influenza sufferer is less likely to experience a sore throat or blocked nose.62 Children, and less commonly adults, might also have gastro-intestinal symptoms.63 While it can be difficult to clinically distinguish between influenza and the common cold, a disease with sudden onset, especially accompanied with fever and the symptoms listed, is more indicative of influenza.
Influenza is often diagnosed on the basis of a clinical history suggestive of the disease rather than by way of biological testing.64 Originally a clinical diagnosis alone was sufficient because, in the absence of any influenza specific treatment, confirmation of influenza in the laboratory did not impact upon treatment of the disease. Today the limited arsenal of anti-viral drugs available for the treatment of
59 It is unknown whether transmission of the virus via conjunctiva results in a respiratory infection;
other routes are known to transmit infection: ibid, summary at Table 8.1, 70.
60 Kirchner in Ryan, above n 18, 73. The question of whether face masks are of value for protection of staff, and if so what sort of mask should be used, will be considered in chapter 7.
61 Generally 38-40 degrees Celsius (perhaps to 41 degrees in children). It is suggested that cough, rather than fever, may be a more reliable indicator of influenza: Leonard A Mermel, 'Influenza Fever Restrictions for Healthcare Workers and Pandemic Planning: Time for Reappraisal' (2015) 36(10) Infection Control & Hospital Epidemiology 1248.
62 Van-Tam and Sellwood, above n 19, and, Kirchner in Ryan, above n 18, 76-81.
63 Kirchner in Ryan, above n 18, 75.
64 Ibid 75.
37 influenza must be taken early in the onset of disease if they are to be of any value.65 Consequently treatment, if implemented, usually commences before testing can confirm the presence of disease.
Rapid influenza tests are in development which may allow identification of the influenza virus within minutes.66 A home testing kit, allowing for identification of influenza A or B, is also in production.67 Given the current limited efficacy of anti-viral drugs, these tests may have little importance in the making of treatment decisions.68 That is not to say home testing would be worthless, rather its value might primarily lie in alerting people to the fact that they are infectious. This knowledge could encourage them to remain at home and ensure they engage in good cough hygiene. Rapid testing might also be used in a workplace to identify infected workers and asking them to go home in order to protect colleagues. However, humans are not the only creatures prone to influenza.