In 2009, an average of 20 percent. the European population was infected at the same time with the seasonal flu and the pandemic H1N1 called swine. However, only 23 percent. these infections showed clear symptoms, and another 17 percent. it ended with visits to the doctor. Despite the mild course of the flu, the complications after it were extremely severe.
These are the findings of a landmark 2009-2010 swine flu pandemic published in The Lancet. A team of researchers from University College London, led by Dr. Andrew Hayward, addressed the key questions: what the flu pandemic really looked like and whether it could happen again in the near future. The cases of this influenza reported by doctors are actually the tip of the iceberg, visible to the national medical risk identification system. It covers only those cases that required medical intervention. Many people missed a doctor even though they had flu. Often, even if they did get him, the flu was not properly diagnosed, one team member said.
We don’t know how many people got sick
The researchers found that the epidemic risk assessment system, based – as in all European countries – on patients of primary care physicians diagnosed with influenza, showed a significant surplus of cases that ended in hospital treatment over the number of patients. In fact, there were many more cases of the pandemic influenza that had been counted, and the deaths and hospitalized patients – far fewer per 100 than estimated to date.
The analysis was performed using the Flu Watch study. It covered five groups of UK households during the six flu seasons 2006-2011. Researchers assessed the prevalence of flu infection, the percentage of infections with specific symptoms, and the percentage of people who received medical attention. People participating in the study provided blood samples before and after each flu season, and a list of people they had contacted during the season, especially if they had flu symptoms such as chills, temperature rises, sore throats and a feeling of general discomfort. Additionally, on the second day of the disease, these people gave nasal swabs to find out what virus they had contracted.
Tests showed that 18-20 percent. the entire unvaccinated population (about 4% of the vaccinated population) contracted seasonal flu during each winter season. In 2008, a slight increase was observed (up to 19-20%), and then in the 2009 season, seasonal and pandemic flu was infected again by 18-20%. population. However, 77 percent. these infections showed no obvious symptoms – people went through them like a cold for several days. In the case of 17 percent. with confirmed influenza of both types, medical assistance was required. Interestingly, the flu from the pandemic season had less severe symptoms than from earlier or later seasons.
The analysis showed that the number of infected people during the pandemic season was 22 times higher than officially stated by the influenza warning system – Sentinel Influenza-Like Illness Surveillance Scheme.
As Dr. Hayward said, perhaps the enormous efforts on the part of European Union countries, expenses and even disruptions in the functioning of companies and offices caused by the imposed security procedures contributed to this development. However, this does not mean that the next flu pandemic will cause milder cases again – scientists need a control and early warning system with a larger population than before to do so. Interestingly, similar results were brought by Swedish research specialists – epidemiologists from the Smittskyddsinstitutet (SMI). According to Swedish epidemiologists, during the pandemic season in 2009, an average of 1 in 6 people across Europe were infected, and in Sweden – 1 in 5. Flu symptoms were also less severe, as was observed by British scientists. However, after this season, there were more post-flu complications, often more severe than with the regular seasonal flu.
The seasonal flu protects somewhat against a pandemic
Meanwhile, researchers at the University of Hong Kong found out what caused the pandemic flu infection to have less severe symptoms. As it turns out, prior exposure to seasonal A flu, and passing it, even the mildest, in a cold-like form, resulted in cross-resistance, that is, there was an immune response to the pandemic, swine flu virus. So the infection couldn’t develop quickly.
Hong Kong researchers also found that in China, most evident cases of pandemic flu – even in the most vulnerable groups such as children and adolescents – showed very slight symptoms of infection. However, the seasonal influenza A vaccines used previously did not provide cross-resistance to the H1N1 pandemic virus, suggesting that exposure to or transmission of influenza A during this season was causing cross-resistance induced by immune memory cells. As previous studies have shown, similar cross-resistance may apply to proteins found in viruses, the so-called H5N1 avian influenza, which would indicate that people who have come into contact with seasonal influenza A during the period of H5N1 virus activity may, during its possible epidemic, experience this flu more easily than patients infected with such a virus for the first time.
According to researchers at the University of Hong Kong, this also means that people who got vaccinated against seasonal influenza A during the H5N1 epidemic or the H1N1 pandemic have the possibility of less likely to develop these pandemic flu viruses, even though they do not have antibodies to their viruses.
But having a pandemic flu more easily does not mean that they are healthy. Conversely, often seemingly quick recovery leads to risky behavior and causes much worse consequences than in the case of medium or medium-severe flu. As noted by the Swedish scientists from SMI and British scientists from Imperial College London, after the pandemic flu season, the number of diseases that are complications of influenza has increased significantly. They were mainly bronchitis, laryngitis and pneumonia, otitis media, but also pericarditis. According to Dr. Hayward, the pandemic flu, whether mild or moderate in terms of symptoms, required rest and lounging in bed, while people with mild flu continued their normal lifestyles, often working hard. Thus, they weakened the body and facilitated the occurrence of complications.
Worse still, scientists from another team at Imperial College London, led by Prof. Tena Feizi found that the pandemic H1N1 virus, unlike the type A seasonal influenza viruses, penetrates cells deep in the lungs. This means a greater risk of developing respiratory complications, even with mild flu, if not treated properly.
Tekst: Marek Mejssner