**<u>Table 1.</u>** **_<u>Determinants of the Base Infectivity of Influenza Virus Strains</u>_**

|_Characteristic_|_Conclusions for seasonal influenza A_|_Implication fo_<br>_epidemic control _<br>_r_|_R_<br>_w_<br>_study source_<br>_evie_|
|---|---|---|---|
|Virus<br>shedding rate<br>& duration|�<br>ry<br>High virus load in respirato<br>�<br>secretions<br>�<br>ms<br>Shedding up to 24‐48 h_before_sympto<br>Virus shed for up to 5 days<br>� Volume & duration higher in children|Symptom‐based<br>control insufficient.<br>be foci<br>.<br>Schools may<br>of transmission|Bell_et al._<br>2006, Tellier<br>2006<sup>28</sup>|
|Timing o<br>peakshedding<br>f|� First 1‐3 days since symptoms (illness)|More rapid<br>transmission*|Bell_et al._<br>2006<sup>55</sup>|
|Mode of<br>transmission<br>(rankorder)|1. Virus‐laden large droplet in cough/sneez<br>2.<br>3.<br>e<br>Virus‐laden small aerosol in cough/sneeze<br>Direct contact with secretions, fomites**|Surgical face masks<br>may not offer<br>complete protection|Tellier 2006|
|Survivability|� Infectious for <8‐12 h from tissue/cloth<br>� Infectious for <24‐48 h from steel/plastic<br>� Humidity,heatreduce survivabilityinair|transport a<br>of infection<br>Malls and crowded<br>source|2006<sup>13</sup><br>Bell_et al._<br>2006, CDC|

* In comparison, SARS has peak infectivity 5‐10 days after onset of symptoms. ** Any inanimate object that transfers the virus person to person, e.g. dust particles in a sneeze.

Table 1 presents conclusions from recent review studies of the basic infectivity of influenza A viruses, themselves based on the literature on seasonal (interpandemic) epidemics in the West. Any deficiencies in this literature are noted in the review studies cited in Table 1. If it is assumed that a pandemic capable virus would have similar properties, then some implications for pandemic control can be gleaned. These will be later put into the context of specific policies for local/community or international preparedness. It can be noted that some aspects of infectivity for the influenza A virus seem higher than the SARS coronavirus from 2003. The caveat should be reinforced that Table 1 is based on historical studies of seasonal influenza A/B, across many different subtypes. Any future pandemic‐capable strain may have different properties.

An additional characteristic of infectivity is the presence of age‐related patterns in attack rates. Based on data from the US, attack rates were much higher in children than in adults during the 1957‐58 (subtype H2N2) pandemic, but were equalized in the 1968‐69 (subtype H3N2) pandemic<sup>3</sup> . It is unclear if the 1957‐58 pattern was driven by the variation in basic infectivity across age groups (of which there is some evidence vis‐à‐vis viral shedding in seasonal influenza), by subtype‐specific properties, or by the variation across ages in modifiers of basic infectivity such as contact rates. It may be difficult to predict an age pattern of attack rates in a future pandemic, though such a prediction would help setting prevention priorities by age group and locations (e.g., schools vs. offices).

**_Virulence_** : The base virulence of a pandemic influenza virus refers to its ability to cause serious health outcomes in an untreated clinical case of the disease. Given virulence, the likelihood of severe outcomes in a case depends on individual level factors (e.g., a complicating comorbidity), and the use and efficacy of pharmaceuticals in treatment. There are several operational measures of basic virulence based on the particular outcome, e.g., deaths per hundred cases or case fatality rate (CFR), hospitalizations per hundred cases, etc. Virulence is directly connected to the ultimate impact on individuals and the economy if an outbreak becomes an epidemic. In theory, virulence is also connected to the duration and attack rate of an epidemic in the no intervention scenario. If a virus incapacitates or kills those infected so rapidly that they do not transmit it to many susceptibles then the epidemic grows slowly. In modeling, this is shown by holding the infectivity constant and increasing the death rate, which usually increases the total duration of a wave of the epidemic, and

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