RESEARCH ARTICLE- 2007 NIH et al, Public health interventions and epidemic intensity during the 1918 influenza pandemic, 2007

[Joseph Arabasz MD]

Excerpted 

"virology – the scientific study of viruses

began in the closing years of the 19th century. Although Louis Pasteur and Edward Jenner developed the first vaccines to protect against viral infections, they did not know that viruses existed. The first evidence of the existence of viruses came from experiments with filters that had pores small enough to retain bacteria. 

In 1892, Dmitri Ivanovsky used one of these filters to show that sap from a diseased plant transmitted virus to other plants despite having been filtered. 

Martinus Beijerinck called the filtered substance a "virus" and this discovery is considered to be the beginning of virology.

The subsequent discovery and partial characterization of bacteriophages by Frederick Twort and Félix d'Herelle further catalyzed the field, and by the early 20th century many viruses had been discovered. In 1926, Thomas Milton Rivers defined viruses as obligate parasites. Viruses were demonstrated to be particles, rather than a fluid, by Wendell Meredith Stanley, and the invention of the electron microscope in 1931 allowed their complex structures to be visualized.

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1918 flu epidemic, pandemic

NonPharmaceutical Interventions (NPIs)

"Discussion 

     Comparisons across 17 U.S. cities show that the first peak in excess motality ratesduringthefallwaveofthe1918influenza pandemic was reduced in cities that implemented multiple NPIs to control disease spread early in their epidemics than in citiesthatmadesuchinterventionslateornotatall.

This finding suggests that such interventions may be capable of significantly reducing the rate of disease transmission so long as they remain in effect. 

If NPIs weremaintainedindefinitelyoncetheywereputinplace, onewouldexpectthatearlyinterventionswouldbeassociatedwith a reduction in both the peak incidence (and therefore peak mortality rate) and also in the cumulative incidence or cumulative excess mortality rate. 

     However, NPIs used in 1918 did not last indefinitely; rather, most of the NPIs in the study cities appear to have been relaxedwithin2–8weeks,whereasopportunitiesforreintroduction and transmission of the pandemic virus extended for many months. If highly effective NPIs are put in place early in the epidemic, and these result in a smaller epidemic, then a large proportion of the population will remain susceptible to the renewed spread of the virusonceinterventionsarerelaxed. Intheabsenceofaneffective method of otherwise inducing immunity in the uninfected population (i.e., a well matched vaccine), such an epidemic is likely to have  phases, with the first phase mitigated by NPIs and the second commencing after NPIs are relaxed. 

In our review of 17 cities, we observed that cities that implemented NPIs sooner had lower peak mortality ratesduringthefirstwaveand were at greater risk of a large second wave. These cities also tended to experience their second waves after a shorter interval of time. 

As described above, no city in our analysis experienced a second wave while its main battery of NPIs was in place , and second waves occurred only after the relaxation of NPIs

Finally, an important practical issue that requires further study is the question of when such interventions can be relaxed. The implication of patterns observed in the timing and severity of second waves in 1918 seems clear, however. In the absence of an effective vaccine, cities that use NPIs to mitigate the impact of a pandemic remain vulnerable. In practice, and until emergency vaccine production capacity increases, this means that in the event of a severe pandemic, cities will likely need to maintain NPIs for longer than the 2–8 weeks that was the norm in 1918

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     However, nowadays, it would appear that the 80% of those who have minimal symptoms, if any at all, should work, with proper precautions for those at risk