The first case of influenza A (H1N1 or swine flu) was reported in Mexico in March 2009. According to the US Center for Disease Control and Prevention (CDC) that the virus was detected in the US in April, and from there it quickly spread to the rest of the world. A study published in the Indian Journal of Medical Research in 2012 explains that by June 2009, swine flu had spread to 74 countries with 30,000 confirmed cases.
Old pandemic, new results
Given that the world is currently in the middle of another pandemic, which is much larger in scale, the rate of spread of swine flu might not seem overwhelming. But like in the case of the COVID-19 pandemic, epidemiologists predicted second waves of the swine flu pandemic during the autumn months. The CDC says the US did see a second wave of swine flu infections in fall of 2009, but the same was not true for many European nations.
According to a new study published in The Lancet Microbe, post swine flu pandemic studies suggested that the further spread and second wave of the 2009 pandemic of H1N1 virus may have been stopped in Europe by the annual rhinovirus epidemic. In case you are wondering, the annual rhinovirus epidemic refers to the yearly spread of common cold cases during the autumn months of September to mid-December. This spurred the researchers behind this new study into exploring whether the spread of one virus — particularly the common cold — can stop or block the spread of other viruses.
Can one viral infection block the other?
The researchers, based out of Yale University, conducted a clinical data analysis to investigate the co-occurrence of rhinovirus (which is the most common viral infectious agent that causes upper respiratory tract infections, especially the common cold, in humans) and the influenza A virus at the Yale-New Haven Hospital. They also did an experimental infection study to investigate the interference between the two viruses.
Respiratory specimens from adults were taken and analysed over three consecutive winters between 2016 and 2019. The researchers collected 8,284 respiratory samples positive for either rhinovirus or the influenza A virus. To understand how rhinovirus infection affects a subsequent influenza infection, the researchers injected rhinovirus into the epithelial cell cultures developed from the respiratory samples collected from the participants. Three days after the rhinovirus infection took hold, they inoculated the same cultures with the influenza A H1N1 virus. They then used a reverse transcription-quantitative PCR test three days later to check for interferon-stimulated gene responses and RNA expressions, which would imply that antiviral immune responses in the human body are triggered.
Of all the samples that met with the criteria of the study, 989 had rhinovirus infection and 922 had influenza, and the researchers found very low odds of co-detection or the existence of both infections at the same time. The researchers found that the rhinovirus infection in the cell cultures induced the expression interferon-stimulated gene responses, which then protected against the influenza virus after three days. This led to an approximated 50,000 fold decrease in influenza viral RNA, proving that the rhinovirus infection blocks the influenza infection.
The study ultimately proves that one respiratory virus can block the infection from another respiratory virus through the stimulation of antiviral defences in the mucosa of the airways. This also supports the hypothesis that the occurrence of the rhinovirus epidemic in fall 2009 disrupted the spread of the swine flu pandemic in Europe. The researchers, therefore, recommend that since viral interference of this type can potentially affect the course of a pandemic or an epidemic, these findings should be taken into account while designing interventions and treatment protocols for the seasonal influenza and even the COVID-19 pandemic.
For more information, read our article on Swine flu.
Health articles in Firstpost are written by myUpchar.com, India’s first and biggest resource for verified medical information. At myUpchar, researchers and journalists work with doctors to bring you information on all things health.