Coronavirus Cross Immunity: Why Swine Flu Was Milder

Coronavirus Cross Immunity: Why Swine Flu Was Milder

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Is there undiscovered immunity to SARS-CoV-2?

The coronavirus pandemic is often compared to the 2009 H1N1 swine flu. At that time, a severe worldwide wave of illnesses with numerous deaths was predicted, but it was then much milder than expected. How did this rough misjudgment come about?

Professor Dr. Christian Drosten, Director of the Institute of Virology at the Charité in Berlin, recently explained in the NDR podcast "Coronavirus Update" how swine flu was initially predicted to be a deadly pandemic in 2009, but then turned out to be unexpectedly mild . Many people think that the SARS-CoV-2 coronavirus could be the same. With swine flu, however, the conditions were completely different, as the coronavirus expert Drosten shows.

Swine flu false alarm

The swine flu in 2009 was much milder than initially predicted by the experts. “No more patients died from it than from normal seasonal influenza, but not less, as is sometimes shown,” said Drosten. Today you know exactly why you made a mistake back then. Swine flu was by no means completely harmless, but there were no known effects at the time that made the pandemic so mild.

The elderly were resistant to swine flu

Animal experiments at the time gave the impression that the H1N1 virus could be far more dangerous to humans than it turned out to be. "And what you only saw after many months was something that was surprising, and you can see from the incidence data that the elderly in the population do not get so seriously ill," says Drosten. The people suspected of being at risk were surprisingly resistant to the virus.

Certain groups of the population had background immunity

Research on this observation then showed the reason: “It has been seen that those patients who were of a certain age had background immunity - both at the cellular level and later measurable with more precise tests that were carried out , even visible at the antibody level, ”summarizes the virologist. Nowadays one only knows where this background immunity came from.

How did this unexpected background immunity come about?

To explain how the unexpected immunity came about, one has to go back to the Spanish flu in 1918. This pandemic influenza was also caused by an H1N1 virus. "This H1N1 virus circulated until 1957," explains Drosten. The virus was then replaced by an H2N2 virus, the so-called Asian flu, which then prevailed until 1968 and was then replaced by the Hong Kong flu H3N2. "This H3N2 virus still circulates today," said the virologist.

In 1977 there was another small pandemic with an H1N1 virus - the so-called Russian flu. "This H1N1 virus is identical to the H1N1 virus of the Spanish flu and its successors, which circulated between 1918 and 1947," explains the virus specialist. This virus was in circulation until 2009.

H1N1 virus provided cross protection

The Spanish flu and the Russian flu were similar to swine flu and placed cross immunity in certain population groups. This has an effect that epidemiologists call "original antigenic sin" (original antigenic sin). Drosten explains that this effect means that the first-ever influenza to be infected is most firmly anchored in immune memory - for the rest of life.

"Looking back from 2009, we now had two groups in the population who had an immunological memory against H1N1," concludes the virologist. The most important group is the group that suffered their first influenza disease from viruses that were the direct successors of the Spanish flu. All those older than 51 in 2009 were most likely already in contact with the H1N1 virus, which allowed immune memory to respond effectively to swine flu.

There was also a group of younger patients who also came into contact with H1N1 viruses as a result of the Russian flu. These complex relationships, which only became apparent later, were underestimated at the time or were not known at all.

Could there be such cross immunity in corona viruses too?

Drosten emphasizes that it is currently not known exactly what happens to the cross protection of the coronavirus SARS-CoV-2. After all, there are other less dangerous corona viruses that circulate during the flu season. Theoretically, it is possible that there is also some background immunity in SARS-CoV-2 in some people. "New studies are coming out these days that again present cross protection data, which also suggest that there is a bit of cross protection, but certainly not, I dare say that now, certainly not to the extent that it was evident at the time the 2009 H1N1 pandemic was the case, ”summarizes Professor Drosten. (vb)

Read also: Spanish flu - history, causes and symptoms.

Author and source information

This text corresponds to the specifications of the medical literature, medical guidelines and current studies and has been checked by medical doctors.

Graduate editor (FH) Volker Blasek


  • NDR: Coronavirus update (42) with Christian Drosten: Everything came different with swine flu (published: 19.05.2020),
  • Wong, Jessica Y .; Kelly, Heath; Ip, Dennis K. M .; u.a .: Case Fatality Risk of Influenza A (H1N1pdm09): A Systematic Review; in: Epidemiology, 2013,

Video: Coronavirus outbreak: Lessons from the 1918 Spanish Flu pandemic - Wait Theres More podcast (July 2022).


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