"Average adult catches virus just once every five years," the Daily Mail reports.
A study has estimated that influenza infections become less frequent with age and occur every five years from the age of 30.
The study analysed blood samples from volunteers in southern China, looking at antibody levels against nine different strains of influenza that circulated from 1968 to 2009. Using complex mathematical models, researchers estimated the frequency of influenza infections and how immunity changes over a lifetime as people encounter different strains of the virus.
The "twice in a decade" figure may sound surprisingly low, but it is only an estimated average for influenza A. It does not include infections with strains of influenza B or C. Also, the estimate is based on a small sample of just 150 people with an age range from seven to 64. Results may well differ in other countries.
It is important not to be complacent as flu can be dangerous. Precise figures are hard to come by, as flu is often a factor in increasing the risk of fatal complications, rather than a cause of death. A 2013 study estimated that flu was implicated in around 13,000 elderly deaths in England and Wales during the flu season from 2008 to 2009.
Where did the story come from?
The study was carried out by researchers from the London School of Hygiene and Tropical Medicine, Imperial College London and University of Liverpool in the UK; Johns Hopkins Bloomberg School of Public Health in the US; the University of Hong Kong; and Shantou University and Guangzhou No 12 Hospital, in China.
It was funded by the Medical Research Council, the National Institute for Health Research and the Wellcome Trust in the UK; and Fogarty International Centre, the Department of Homeland Security and the National Institute for General Medical Sciences in the US.
This was a highly complex scientific paper (a sample quote – "Hence the titre μ was scaled by a factor s1(X, j) = (1 + τ1)|X"), so unsurprisingly, the media focused on the simple message that according to this study, flu is far less common than many people think. The Daily Mail also reported that "man flu" may be a myth, with no evidence that men are more likely than women to be "struck down" by the bug. The study itself does not look at rates of infection for each sex.
What kind of research was this?
In this study, scientists aimed to look at how our immunity to flu – specifically to influenza A strain (H3N2) – changes over a lifetime as we encounter different strains of the virus. It is important to understand this they say, because how the immune response develops influences the emergence of new strains of the virus, the size and severity of flu epidemics and the effectiveness of vaccination programmes. They say that factors that shape the human immune response are poorly understood, since individual infections and the development of immunity over a lifetime are rarely observed directly.
The immune system responds to flu viruses by producing antibodies that specifically target proteins on the virus surface. These proteins can change as the virus evolves, but we keep antibodies in the blood that have a memory for strains we have encountered before.
What did the research involve?
There were two parts to this study.
Scientists used data from a survey in southern China that examined people’s antibody levels against nine different strains of influenza A (H3N2) from 1968 to 2009. Participants were selected from five different locations, with 20 households randomly selected from each location. Samples of blood were taken and tested for the presence of antibodies against different strains of flu.
To determine the effect of a lifetime of influenza infections on immunity, scientists developed a mathematical model capturing the specific strains with which an individual has been infected and the corresponding antibody response. They examined whether this was affected by factors such as:
- "cross-reactivity", increased immune response to a new strain due to previous antibody response to a different strain
- "antigenic seniority" – whether strains encountered earlier in life provoked a stronger immune response
What were the basic results?
Their model found that "antigenic seniority" and the reduction in cross-reactivity over time were important components of the immune response.
They estimate that while children on average get flu every other year, infections become less frequent as people get older. From the age of 30 onwards, they estimate that flu infections tend to occur at the rate of about two every 10 years.
How did the researchers interpret the results?
The researchers say that the strains encountered early in life and the order in which individuals were infected with the flu virus influence their immune response, which in turn could shape the evolution of the flu virus. These findings, they argue, could also help us better understand future susceptibility to new strains and develop future vaccination programmes.
This complex scientific study looked at which factors might influence the immune response to flu over someone’s lifetime and also produced an estimate of how frequently people in different age groups are affected by flu. The details are of interest mainly to other scientists involved in studying the flu virus, how it may evolve and the best way to protect ourselves against it.
When considering the results, it is important to note that these are estimates. They are based on blood samples from 150 people. This means there would have been a limited number of people in each age group, which spanned age seven to 64. In addition, the participants were selected from 20 households in each of five study locations in southern China. People living together are more likely to infect each other with the virus, and so the results may be different among other population groups.
The estimates are also based on nine strains that were originally recorded in 1968, 1975, 1979, 1989, 1995, 2002, 2003, 2005 and 2008. It does not cover other strains, influenza B or C, or whether the immune response was due to previous vaccination or infection.
Additionally, the researchers had to make a number of assumptions, which need to be taken into account when considering the results:
- They estimated the number of times people had been infected by each strain by assuming each subsequent infection with the same strain of virus would boost the immune response.
- They considered that the immune response to a new strain would not be as high as to previous strains, with the first-ever infection creating the biggest immune response.