Swine flu latest from the NHS
Up to a third of children in some areas may have had swine flu, but many will not have been ill, analysis shows.
The Health Protection Agency has reviewed blood tests which showed higher levels of infection among children than originally thought.
In hotspot areas, such as London and the West Midlands, a third of school-aged children may have had the virus, but only one in 10 or less got ill.
Across the UK, the figure is probably about a fifth, the HPA said.
The findings reinforce the fact the pandemic is a mild strain of flu.
Flu, whether it is seasonal or swine, always affects more people than actually get ill.
The ratio is normally about 50-50 and pandemic flu is probably not much different, the HPA said.
The agency has been carrying out blood tests on sample groups as part of its on-going surveillance programme to look for signs that individuals had come into contact with swine flu.
These have revealed high levels of infections without symptoms among children - between three to fives times as many have had the virus than have fallen ill.
The revelation is interesting because it means the 1m figure for the overall number of cases seen so far is likely to be an underestimate by some way.
The findings also suggest there may be a significantly higher degree of immunity in children than thought - once someone has had swine flu they cannot get it again unless the virus mutates.
Coupled with this, between 30% to 40% of over 50s are thought to have some immunity from previous strains they have come into contact with.
Swine flu is from the same family of the virus as the normal seasonal flu and previous pandemics.
Impossible to predict
The HPA admitted this could explain why the virus had not really taken off this autumn - the number of new cases is currently half of what they were in the summer.
However, Justin McCracken, chief executive of the HPA, said it was still impossible to predict what would happen next.
He added the latest analysis just gave an idea of what had been going on so far.
"Those that meet the case definition are just the bit of the iceberg above the water. We are giving an idea of how big the iceberg is under the water."
HPA officials also tried to calm fears about the emergence of the drug-resistant strain of flu seen in Cardiff last week.
Maria Zambon, director of the HPA's Centre for Infections, said the revelation was "not unexpected", but she added many mutated strains struggle to replace the original strain as the more dominant one.
"We will have to see if it can outcompete the non-resistant one."
Why are healthy people over 65 and children not a priority for the swine flu vaccine?
Healthy people aged over 65 appear to have some natural immunity to the swine flu virus. And while children are disproportionately affected by swine flu, the vast majority make a full recovery - therefore the experts do not advise that children (other than those in at-risk groups) should be vaccinated initially.
Revised planning assumptions
It was announced on September 3 that estimates of deaths in the worst-case scenario for swine flu have been lowered. The government's expert advisers on swine flu, the Scientific Advisory Group for Emergencies (SAGE), said that new data from the UK, north America, Australia and elsewhere give a better picture of how the virus might spread in the autumn.
The revised planning assumptions have cut the estimated death toll in a worst-case scenario from 65,000 people in the UK to 19,000, assuming that 30% of the population is infected.
These forecasts and others in the report are based on a "reasonable worst case" value and should not be taken as a prediction of how the pandemic will develop. Planning against the reasonable worst-case scenario will ensure, however, that plans for all likely scenarios are robust.
The Department of Health said: “In light of this new information, the estimates for the number of people who might need hospitalisation and the proportion of people with swine flu who could die have been reduced.”
Vaccination should cut hospitalisation
These new planning assumptions do not take account of the vaccination programme which, once it has begun, will help to further reduce the number of people needing hospitalisation. However, the department added, we must not be complacent. While in the majority of people it is mild, for some this virus can be a serious illness.
H1N1 Swine Flu traced to 1918 pandemic:
The virus responsible for the Spanish flu in 1918 created a ‘viral legacy’ that continues to this day, according to a study published recently in the New England Journal of Medicine.
According to the authors of the report, the Spanish flu's H1N1 virus, which caused tens of millions of deaths in 1918, was also transmitted from humans to pigs during the pandemic. Tracing the lineage of the virus in this research shows that it continues to evolve in both humans and pigs 90 years later.
All human-adapted influenza A viruses "are descendants, direct or indirect, of that founding virus" says Jeffrey Taubenberger, a co-author of the report and a senior investigator at the Laboratory of Infectious Diseases of the National Institute of Allergy and Infectious Diseases in the US.
Key points from this research
- All flu viruses contain eight genes in total, including two that contain instructions for producing the proteins hemagglutinin (H) and neuraminidase (N), which allow the virus to attach to a host cell and spread from cell-to-cell.
- There are 16 sub-types of the H protein and nine sub-types of the N protein a flu virus can possess. This offers 144 possible HN combinations, but to date, only three (H1N1, H2N2 and H3N2) have been observed to be fully adapted for infecting humans.
- There are other combinations, such as H5N1, a strain of bird flu virus, but these have only occasionally infected a small number of humans.
Where was the article published?
This article was written by D M Morens and colleagues from the National Institute of Allergy and Infectious Diseases, Bethesda, in the US. It was published in the New England Journal of Medicine. No potential conflicts of interest were reported.
A related article on the evolution of the current pandemic strain of the H1N1 virus was also published in the same issue and has been covered in Behind the Headlines.
What kind of study was this?
This was a review article written by acknowledged experts in the field, explaining the lineage of the pandemic flu virus seen in 1918 and relating it to the emergence of the pandemic H1N1 strain currently circulating.
What do the researchers say?
The researchers explain that descendents of the H1N1 influenza A virus that caused the pandemic of 1918–1919 have persisted in humans for over 90 years, and have continued to contribute their genes to new viruses that have caused epidemics, new pandemics and epizootics (epidemics in animal populations).
The current pandemic strain is thought to originate from two unrelated swine viruses, including a derivative of the 1918 human virus. It also appears to contain genes from human, bird and swine flu viruses. The authors sought to detail the family history or ‘lineage’ of this virus, mapping out the complex relationship between a number of different strains that may have exchanged genetic material.
The authors created an analogy to explain how genetic material transfers and mutates. They said it is helpful to think of influenza viruses as a team of eight genes working together. Occasionally the viruses ‘trade’ one or more team members to make way for new genes, or ‘players’. These new players bring with them ‘unique skills’, and through trading genes in this way (called ‘shift’) and by accumulating mutations (called ‘drift’) the influenza viruses are able to change and evade the immune system.
The authors also researched the mortality rates in seasonal epidemics and previous pandemics, expressing doubt over the claim that gene shift always cause severe pandemics while drift leads to more modest increases in seasonal mortality.
The authors raised several more interesting points in their article:
- Influenza pandemics over several centuries have shown a big variation in severity, ranging from mild to severe.
- The new swine flu virus is a fourth-generation descendant of the 1918 virus.
- It appears that successive pandemics and pandemic-like events generally appear to be decreasing in severity over time. They say this is probably due to medical and public health advances.
What is the implication and importance of this?
This report helps to explain the evolution of the current pandemic strain of influenza virus, an area where extensive study can be expected. Although the genetic code of this virus has already been sequenced, this type of research may help in the search for effective vaccines, which remain the best hope for minimising the complications expected.
The authors say that while “we must be prepared to deal with the possibility of a new and clinically severe influenza pandemic caused by an entirely new virus, we must also understand [the current pandemic] in greater depth” and continue to “explore the determinants and dynamics of the pandemic era in which we live."
