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Humans have passed swine flu to pigs 370 times since 2009

A recent study published in the journal PLOS Pathogens has found that the strain of influenza A that triggered the 2009 swine flu pandemic – known as “pdm09” – has been transmitted from humans to swine about 370 times since 2009. 

The subsequent circulation in swine has resulted in the evolution of a variety of pdm09 variants that spilled back from swine to humans. 

Focus of the study

Influenza A is known to cause respiratory illnesses in humans, swine, birds, and other mammals. In 2009, a pandemic caused by the pdm09 strain led to thousands of human deaths around the globe. Since then, this strain has been repeatedly passed from humans to swine, leading to evolutionary changes that make it more likely to cross back and infect humans.

To clarify the risks emerging from this widespread circulation of the virus, the researchers analyzed pdm09 transmission data between 2009 and 2021, and investigated how these interspecies transmission events have affected the genetic diversity of the virus and the risks of further human infections.

What the researchers learned 

The analysis revealed that, since 2009, this strain has passed from humans to swine approximately 370 different times, with most of these events occurring when the pdm09 burden was the highest among humans. 

Although during the first two years of the Covid-19 pandemic, circulation among humans dropped, pdm09’s circulation in swine persisted due to about 150 human-to-swine transmission events from 2018 to 2020.

Most human-to-swine transmission events were isolated, but some of them led to sustained circulation of various pdm09 genetic lineages among swine in the United States. 

Implications of the study 

Since these variants were shown to be genetically poor matches for human seasonal vaccines, even widespread vaccination would have provided little protection against them. This is particularly worrisome since persistent circulation of this strain among swine was linked to at least five instances of swine-to-human transmission.

These findings suggest that managing infections in people who work with swine on a regular basis could help prevent transmission to pigs, and thus reduce the risks associated with the virus passing back to humans.

“Controlling influenza A virus infection in humans can minimize spillover of viruses into pigs and reduce the diversity of viruses circulating in swine populations. Limiting virus diversity in pigs can minimize the emergence of novel viruses and the potential for swine-to-human transmission of influenza A virus,” the authors concluded.

Swine flu pandemic 

The swine flu pandemic of 2009 was caused by a new strain of H1N1 influenza virus. The World Health Organization (WHO) declared the outbreak a pandemic on June 11, 2009, as the virus had spread to over 70 countries.

Originating in Mexico in April 2009, it quickly spread globally due to the high transmissibility of the virus. The pandemic affected all age groups with higher illness and hospitalization rates among children and young adults. Older adults were somewhat protected due to likely exposure to similar virus strains in their past.

By August 2010, when the WHO declared the pandemic over, it had spread to 214 countries, with over 18,000 confirmed deaths. However, some estimates place the actual number much higher, as many deaths likely went unreported or unrecognized. The majority of deaths were in younger people, a departure from seasonal influenza, which typically affects the very young and very old most severely.

The 2009 pandemic highlighted the importance of rapid global health communication, flexible public health infrastructures, and comprehensive pandemic planning, including vaccination development and distribution strategies.

More about swine flu

The term “swine flu” was first used to describe a flu pandemic in 2009, which was caused by a novel strain of H1N1 that had not been previously seen in humans or animals. This was a unique combination of influenza viruses typically found in pigs, birds, and humans.

Swine flu symptoms are similar to those of other influenza strains: fever, cough, body aches, fatigue, chills, and in some cases, severe illness and complications such as pneumonia and respiratory failure can occur.

Swine flu is contagious and spreads in the same way as the seasonal flu. When people who have it cough or sneeze, they spray tiny droplets of the virus into the air. If you come in contact with these particles, touch a surface (like a doorknob or sink) where they’ve landed, or touch something an infected person has recently touched, you can catch H1N1 swine flu.

Vaccination is the most effective way to prevent infection. Antiviral drugs are also available that can help treat the flu, including swine flu. These work by preventing the flu virus from reproducing in your body. They are most effective when taken within 48 hours of the onset of symptoms.


By Andrei Ionescu, Staff Writer

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