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Congressional Briefing: Novel G4 H1N1 Influenza

Updated: Feb 8

By Ravi Starzl

The high degree of national vulnerability to infectious respiratory disease observed during the COVID-19 pandemic brings into focus the need for rapid identification of similar emerging biothreats, to both enable early containment and development of countermeasures. The H1N1 influenza virus represents just such a threat – it is a known killer with a deadly history that possesses the ability to evolve serotypes that are both highly infectious and for which there is no established herd immunity. The possibility of a widespread H1N1 outbreak during the COVID-19 pandemic would compound the strain on regional medical systems, as there is no cross-protection between COVID-19 and H1N1 influenza immunity.

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H1N1 is an influenza A subtype whose strains had been responsible for several known major flu epidemics, including the deadliest known flu pandemic in 1918-19. Different strains of the H1N1 virus are endemic to humans, pigs, and birds.

There have been three pandemics of influenza in the 20th century and one in the 21st century (Kilbourne, 2006). The H1N1 virus caused a significant and best-known pandemic in 1918-19 and the most recent one in 2009-10. In addition, there were three other notable epidemics in the 20th century that were not considered pandemics: (i) a pseudo pandemic in 1947 (that was not considered pandemic due to low death rates), (ii) 1977 epidemic that was pandemic in children, (iii) an abortive epidemic of swine flu in 1976 in Fort Dix, NJ. All of these were caused by strains of the H1N1 virus.

An antigenic variation of H1N1 caused the 1947 epidemic. One remarkable feature of this epidemic was the complete failure of the H1N1 vaccine with the 1943 H1N1 strain due to the antigenic drift of the 1947 virus. Millions of U.S. military personnel that were vaccinated were found to have no protection against the new strain of the H1N1 virus. Luckily, the epidemic caused relatively few deaths and is, therefore, considered to be a pseudo pandemic.

The 1976 outbreak of H1N1 was confined to Fort Dix, NJ, although it triggered a mass vaccination program that planned to vaccinate 43 million Americans. The vaccination was suspended after repeated reports of Guillain-Barré syndrome affecting vaccinated individuals in around a dozen states and seven times the higher reported incidence of swine flu in vaccinated individuals. There were indications that reporting bias was to blame for at least part of these cases, as there were no specific tests for Guillain-Barré syndrome, and the doctors were aware of the reported link between

the vaccination and the syndrome. In any case, the vaccination program was not reinstated, and the virus did not spread. This outbreak became a cautionary tale about overreacting to the danger of an epidemic by initiating a pre-emptive vaccination program with a hastily developed and tested vaccine.

In 1977, there was an outbreak of the 1947 H1N1 virus in the Soviet Union, which spread worldwide. Because the older population was already exposed to the strain in 1947-1957, the epidemic primarily affected the population under 25. Since both haemagglutinin (HA) and neuraminidase (NA) antigens were very similar to the 1947 strain, this lack of antigenic drift led to speculations that this strain of H1N1 virus escaped from a laboratory somewhere in the USSR.

The latest 2009-10 pandemic caused around 500,000 cases and 18,500 deaths, although it has been speculated that these numbers are significantly higher: CDC estimated around 284,000 (Roos,2012). For comparison, reported deaths from influenza worldwide are 200,000-500,000 annually. Unlike most influenza viruses, this strain of the H1N1 virus did not affect disproportionately people over 60 years old.

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