Wednesday November 28 2007
Flu is spread via airborne droplet infection
“Simple measures ‘may thwart flu’,” reported BBC News today. BBC News says that “researchers found handwashing and wearing masks, and gloves and gowns all had a positive effect – and were even more effective when combined” and these could be effective in preventing the spread of flu if there was a pandemic. This research will lend weight to the new Department of Health campaign: ‘Catch It, Bin It, Kill It’. Although this doesn’t advise wearing face masks and gowns, it stresses the importance of using a tissue to catch coughs and sneezes, throwing the tissues away immediately after use and washing your hands as soon as possible.
The news story is based on research that has assessed the effectiveness of physical preventative measures to halt the spread of viral infection. This research is particularly important because of recent reports about outbreaks of avian flu and the potential for future episodes of pandemic flu in the UK. There are concerns that, even though vaccines and antiviral drugs may be prepared for future outbreaks, these may be ineffective. As respiratory infections such as flu are spread via airborne droplet infection and can be passed via contact from hand and mouth, physical barrier methods can be effective in preventing infectious spread.
Where did the story come from?
Tom Jefferson of the Cochrane Vaccines Field, Italy, and colleagues from the Public Health Agency of Lagio Region, Rome; the University of York; Bond University, Queensland, Australia; and Manipal Academy of Higher Education, India, carried out this research. The study was funded by Cochrane Collaboration Steering Group, UK, and each author’s institution. It was published in the peer-reviewed British Medical Journal.
What kind of scientific study was this?
This was a systematic review in which the authors looked for evidence about the effectiveness of physical interventions in preventing the spread of respiratory infections.
The researchers searched through a number of medical databases to identify trials that had looked at any intervention to prevent the spread of respiratory infection between humans or between humans and animals. These interventions included personal hygiene, physical barriers, isolation or quarantine, but the researchers excluded trials with medical interventions, such as antiviral drugs or vaccines. They included randomised and non-randomised controlled trials, case-control and cohort studies, which demonstrated some evidence of trying to control for possible confounding factors that could be having an effect on the spread of infection.
The researchers combined study data where appropriate, accounting for differences in study design, the types of exposure to infection, the differences in the populations examined, and the observed results. Where possible, they performed calculations to give an overall measure of the size of effectiveness of the intervention. In total, 51 studies were included in the analysis and these included different types of intervention, from hand washing and educational programmes to prevent the spread of respiratory illness in children, to the isolation of healthcare workers caring for people with suspected SARS virus in Hong Kong.
What were the results of the study?
The researchers found that the studies included in their analysis varied widely in quality and methods, and some of the problems encountered included inappropriate study design, loss of data, inadequate blinding, bias and difficulty with participants sticking to the interventions. Most of the study results could not be combined statistically, due to differences in methods.
The researchers found that the best evidence for the effectiveness of physical measures came from one randomised controlled trial (RCT) investigating hygiene measures to prevent spread of infection in young children, and six case-control studies investigating the impact of public health measures to curb the spread of the 2003 SARS epidemic in China, Singapore and Vietnam. The RCT demonstrated that risk of respiratory illness was significantly reduced by hygiene measures in children aged up to two years. The combined results of the six case-control studies found that hand washing more than 10 times daily, wearing masks (both conventional surgical masks and those with special filters to prevent spread of droplets), wearing gloves, or wearing gowns, either done singly, or all together reduced the spread of the SARS virus. The studies used in the analysis did not allow the researchers to evaluate the effects of global measures, such as screening at ports of entry into countries.
What interpretations did the researchers draw from these results?
The researchers concluded that simple measures such as hand washing and wearing face masks can help prevent the spread of viral respiratory infections. They acknowledge the difficulty in ensuring that adequate trial conditions were used to test these interventions, such as participants being unaware of intervention, and preventing bias, or poor reporting; however, despite not being able to reach firm conclusions from their findings, they say their systematic review “provides some important insights.” They recommend simple, low-cost interventions and say that “further large pragmatic trials are needed to evaluate the best combinations.”
What does the NHS Knowledge Service make of this study?
Although the studies identified by this piece of research are of very variable quality, the interpretation and methods used by the authors when combining the studies in their systematic review are very reliable. As respiratory infections such as flu are spread via airborne droplet infection and can be passed via contact from hand and mouth, physical barrier methods can be effective in preventing infectious spread. However, determining exactly how effective these measures are is always going to be quite difficult, as the authors found. Some of the problems that they encountered when trying to combine the results of the studies identified were:
- The populations and settings used in the studies were different, ranging from children in the community, to hospital intensive care units, to army barracks. There were few studies from the developing world, and since these countries have the greatest burden of infectious disease, they would probably benefit most from simple, cheap interventions.
- There were several problems with participants sticking to the interventions, preventing bias, the consideration of other factors that may have an effect on spread of infection, maintaining blinding (almost impossible when you are testing interventions such as hand washing), and testing in a situation that represents what might actually occur in real life, rather than just under experimental conditions.
- It has not been possible to account for how common the virus being tested was among the population at that time. This would have a huge impact on how much at risk a person would be of contracting it.
- The researchers were unable to assess more global barrier methods such as screening at entry into countries and population distancing which, in the event of a worldwide pandemic, would be of particular importance.
This research should inform debate about possible ways to prevent the spread of an avian flu pandemic.