Influenza epidemic
Course and effects of an epidemic
One of the secrets of the influenza virus is its almost total absence in the human population of the Northern and Southern Hemisphere over the major part of the year. And then, suddenly, in an unforeseeable moment during the winter (between October and April in the Northern Hemisphere and between May and October on the Southern Hemisphere), the first sporadic cases of influenza occur and in the course of weeks the number of new cases increases dramatically within a large part of the society. This lasts for 1 to 3 months, and then the virus disappears once again. It is still poorly understood where it stays throughout the major part of the year and how and where it undergoes its antigenic drift over that time. One of the possible hypotheses is that the virus remains endemic (in small quantities throughout the year) in the population of people living on the equator.The chart below presents, as an example, one of influenza epidemics (1983/4, beginning in the Netherlands). After the first isolation of influenza virus in January 1984, the number of new cases of flu-like diseases grows initially slowly (January), and then very rapidly (February). The disastrous effect of the epidemic on absence from work and school, economic losses, etc, is visible in this period. What is more important, mortality among persons in high risk groups (the elderly and patients with chronic diseases) increases.
1983/84 - epidemic in the Netherlands
Bars: Number of influenza virus isolates in all virological laboratories.
Dotted line: Cases of flu-like disease (FLD) per 10,000 inhibitants, based on the raports of research stations in the country.
Continuous line: Number of deaths attributed to influenza, estimated on the basis of the epidemiological methods.
During an influenza epidemic with moderate activity in the European and North American populations, 60,000 do 120,000 persons per million will get symptomatic disease (similarly as in the example in the above chart), 700 do 1,600 persons per million will have to be hospitalised, and 50 to 150 persons per million will die. The table below presents how these incidences translate to several countries. When the epidemic is more serious, these figures may proportionally increase.
Table: Estimated annual morbidity and increased mortality during a moderate influenza epidemic
| Country | Number of inhabitants (in millions) | Flu-like desease | Additional hospitalisations | Additional deaths |
| | | | | |
| The USA | 250 | 15,000,000 – 30,000,00 | 175,000 – 4,000,000 | 12,500 – 37,500 |
| Austria | 8 | 480,000 – 960,000 | 5,600 – 12,800 | 400 – 1,200 |
| Belgium | 10 | 600,000 – 1,200,00 | 7,000 – 16,000 | 500 – 1,500 |
| France | 56 | 3,360,000 – 6,720,000 | 39,200 – 89,600 | 2,800 – 8,400 |
| Germany | 77 | 4,620,000 – 9,240,000 | 53,900 – 123,200 | 3,850 – 11,550 |
| Italy | 55 | 3,300,00 – 6,600,00 | 38,500 – 88,000 | 2,750 – 8,250 |
| Portugal | 10 | 600,000 – 1,200,000 | 7,000 – 16,000 | 500 – 1,500 |
| Spain | 40 | 2,400,000 – 4,800,000 | 28,000 – 64,000 | 2,000 – 6,000 |
| Switzerland | 7 | 420,000 – 840,000 | 4,900 – 11,200 | 350 – 1,050 |
| Netherlands | 15 | 900,000 – 1,800,000 | 10,500 – 24,000 | 750 – 2,250 |
| United Kingdom | 56 | 3,360,000 – 6,720,000 | 39,200 – 89,600 | 2,800 – 8,400 |
Particularly spectacular are influenza epidemic outbreaks in closed or half-closed communities such as schools, military camps, hospitals or nursing homes, due to their very high morbidity. Outbreaks in nursing homes are described in the medical literature every year, with an incidence of even 50% or more, with mortality of 5% or more.
The figures presented in the table above are estimates for a moderate epidemic. It should be noted, however, that a significant and unforeseeable variability exists with respect to the occurrence, duration, intensity (i.e. effect on morbidity and mortality) of predomination of strains and (sub)types (of type A-H1N1, A-H3N2 or B influenza virus) in each winter and at each location.To illustrate this huge variability, the Figure below presents the occurrence and duration of the epidemic in Netherlands in 1971-2 to 1998-99. The mean duration of these 28 seasons was approximately 2 months, but frequent deviations were noted: for example, the total lack of epidemic activity in 1979-80 and 1987-88 (marked by an asterisk), and on the other hand the period of almost five months in 1978-79. The initial month is usually January, but also very early (in 1988-89 and 1993-94: November) as well as very late (in 1981-82 and 1983-84: March) epidemics occurred. Each epidemic has its own dynamics and unique values of morbidity and mortality.
Figure: Beginning and end of the influenza epidemic in Netherlands (the winter season of 1971-2 to 1998-1999)
Abstract:
- In the winter season, the influenza virus appears suddenly in the population, causing epidemics on the country scale and/or local outbreaks.
- General population: morbidity and mortality may be significant (absenteeism in schools and at work).
- Elderly and chronically ill persons: morbidity and mortality may be high.
- Influenza epidemics is associated with a huge personal, medical and economic burden.
- High and unforeseeable variability exists among epidemics with respect to the occurrence, duration, intensity and predominating strains.
