Swine flu is not hyper-virulent. While it spreads relatively easily it does not appear to be any more virulent that the strains we have circulating every year around the world, writes microbiologist Peter Collignon.
There are, appropriately, public health concerns with any new strain of influenza that develops. During the last century we have had three major events where millions of people died after new strains (H1, H2 and H3) spread to people and to which we had little or no pre-existing immunity (eg H1N1 Spanish Flu from 1918-19). These new strains spread to people from strains derived from birds, often after mixing their genetic components in pigs. This latter mixing made them more adept at spreading from person to person. The current “Swine flu” strain has a mix of human, pig and bird genetic components and thus is of some concern.
However this current strain does not fulfil two essential characteristics needed for any Flu strain to be the same problem as Spanish flu. For any Flu strain to be a problem of the same magnitude it needs: to readily spread from person to person AND to have a more aggressive (or virulent) effect in people compared to the strains that are circulating now and causing problems every winter.
This current swine flu strain is not more virulent. It does however transmit readily and so will very likely cause a pandemic when it spreads around the world.
However there is a problem with the word pandemic. This just means that the virus spreads widely across continents. “Pandemic” however seems to invoke a feeling that a new epidemic disaster has hit us and with a strain of virus that will kill a large proportion of a population (ie it is also hyper-virulent).
Swine flu is not hyper-virulent. While it spreads relatively easily it does not appear to be any more virulent that the strains we have circulating every year around the world. The most reliable figures to judge its virulence so far come from the US. Data from developing counties is often incomplete because of poorer testing facilitates and an inability to find and test mild cases. This will lead to an overestimation of the virus’s virulence.
Currently in the US it appears that for every 1000 people who get infected, about 40 people need admission to hospital and about one person dies. This is a still an aggressive virus, but no more so than the Flu viruses that change slightly every year or so, and then circulate around the world, mainly causing problems in winter.
Given this Swine Flu strain is H1, we would expect many in the population to have some immunity because variations of H1 strains have been recirculating in people since 1918. This appears to be the case and is reflected in the relatively small numbers of people over the age of 30 who have been infected. This is quite different to the situation when completely new H strains come across from pigs or birds (eg H7), where almost no one has been previously exposed and thus we have little or any pre-existing immunity.
However with this Swine Flu, even children and young adults, with presumably little or no pre-existing immunity, do not seem to be experiencing excessive mortality compared to seasonal influenza. Again this reflects the relatively low virulence of this current strain. It is argued by some that this strain could change tomorrow and become much more virulent. This it always possible, but so can the strains that recirculate very year now (H1, H3 and influenza B).
We need to also consider what killed most people when new and virulent Flu pandemics spread across the world previously. It is not the Flu virus itself. Most deaths were likely the result of secondary bacterial infections especially Staphylococcus aureus and pneumococcus. The high death rate in 1919 was because there were no antibiotics developed yet. In the late 1950’s (Asian Flu), it was because there was a lack of available and active antibiotics — penicillin resistance had developed and spread rapidly in Staphylococcus aureus by then. Antibiotic resistance is a major and rapidly growing international problem, especially in developing countries.
However in Australia we are fortunate because we still have a variety of antibiotics (especially injectables) that will work against nearly all strains of bacteria that might complicate Flu and cause pneumonia. We also know we can slow or stop the spread of Flu virus, even in households with close personal contact by good hygiene, hand care (alcohol hand rub and soap and water), masks and other general infection control measures.
Overall we need to reconsider how we approach this virus. We have Flu strains every year that proportionately cause similar levels of illness and deaths that what this Swine Flu strain currently is causing. While we need to do all we can to prevent these seasonal infection, I don’t think we should take undue, dramatic and different measures for all new strains. We should only do that if any new influenza strain fulfils or is likely to fulfil, both conditions needed to cause a dramatic and different outcome to seasonal influenza — hyper virulence and easy spread.
Peter Collignon is an Infectious Diseases Physician and Microbiologist and Professor, School of Clinical Medicine, Australian National University.