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1 Overview

The first case of Influenza A (H1N1) occurred on Mar. 2009, when human infection with swine flu epidemic broke out in Mexico. The influenza quickly spreaded throughout the world. The WHO referred it initially as human infection with swine flu, but later nominated it as Influenza A (H1N1). On June 11, 2009, WHO declared to raise its epidemic alarming level to grade VI, signaling its prevailing worldwide. The clinical symptoms of Influenza A (H1N1) were similar to those of regular human seasonal influenza. However, Mexico saw its high mortality rate, quicker spreading rate and susceptibility in general population.

On Apr. 30, 2009, Influenza A (H1N1) was listed as Class B infectious diseases and the frontier health and quarantine infectious diseases in China. It has been managed as the Class A infectious diseases since then [1] (Table 3.1).

Table 3.1 Pandemic distribution of Influenza A (H1N1) (the latest update at 5: 45 on March 2, 2010, UTC)
Full size table

2 Source of Infection

The patient suffering from the disease is the dominant infectious source of the newly pandemic Influenza A (H1N1). Although Influenza A (H1N1) virus had been isolated from swine, there are still no evidence definitely proving swine as the infection source. The incubation period after infection is 2–7 days. Most patients may detoxify from 1 day prior to symptoms onset to day 5–7 after the onset. Some may even detoxify after no symptoms remain. The infectious period may even longer in young children, patients with compromised immunity or seriously ill patients.

3 Route of Transmission

Influenza A (H1N1) spread from person to person through exhaled aerosol and droplets. Because droplets cannot drift in the air, they usually can only spread the virus to persons in short distance from the patients. Influenza A (H1N1) can also be spread via direct or indirect contacts of oral mucous membrane, nasal mucous membrane, eyes mucous membrane. Exposure to respiratory secretions from patients, body fluid from patients and items contaminated by the viruses also can give rise to its spread. In addition, faecal-oral transmission should not be ignored as one route of transmission because many patients have the symptom of diarrhea, indicating their detoxification through stool.

4 Susceptible Population

Generally, all groups of population are susceptible to Influenza A (H1N1). It occurred in people from all ages groups, but with more young adults than the seniors. Many death cases are young adults with good health conditions. Among 642 definitely diagnosed patients in the US from Apr. 15, 2009 to May 5, 2009, 60 % of the patients aged 18 years or younger when they were still school students. It is indicated that children and young adults are more susceptible to Influenza A (H1N1) than the seniors, which probably due to their different social networks. Influenza A (H1N1) showed a delay spread in senior population, which may be due to the protective effects of their previously produced swine influenza virus antibody in the seniors. High-risk groups of Influenza A (H1N1) include medical staffs with contacts to patients, family members of patients, and people who travelled in the epidemic areas.

5 Effects of Natural and Social Factors on Its Spread

5.1 Effects of Natural Factors on Its Spread

The natural factors influencing the spread of Influenza A (H1N1) include climate, geographical factors and the mutual interactions between hosts and viruses.

5.1.1 Host Factors

The age of the host: Age is related to the severity of the infection. It is believed that the young adults have a higher incidence of Influenza A (H1N1), but the seniors and the pregnants are also high-risk groups.

Some experts believe that different racial and ethnic groups have different susceptibility to Influenza A (H1N1), which needs further studies concerning its molecular genetics.

5.1.2 Viral Factors

Viral load: Viruses with higher viral loads have more infectious power.

Viral virulence.

Other factors: The exposure distance to the patients and the exposure time. For example, during close contacts and open respiratory tract operations, especially invasive mechanical ventilation (tracheotomy or open operations, such as spray inhalation or sputum suction), would greatly increase the infectious power of Influenza A (H1N1).

5.2 Effects of Social Factors on Its Spread

The effects of social factors on the spread of Influenza A (H1N1) are usually complex and changeable. The factors include social status, medical insurance, disease prevention and control agencies, residence conditions, economic and cultural status, hygienic habits and customs, religion, social security etc.

The disease prevention and control agencies play a central role in controlling such infectious disease as Influenza A (H1N1). Their emergency responses directly determine its prevalence. The principles for its prevention and control are early detection, early report, early quarantine and early treatment. Outbreak and spread of Influenza A (H1N1) occurs in social conditions of convenient transportation, densely populated area and poorly ventilated room.

6 Molecular Epidemiology

6.1 Molecular Biology of Host Specific Influenza A (H1N1) Virus

Influenza A (H1N1) virus characteristically spreads from one species to another. During the viral infection, HA plays a role in recognizing and adsorbing cell receptors, which serves as the major determinant of the host specificity. The cell receptors in the tracheal mucosa of different animals specifically bind to receptor binding sites in the HA molecules. And HA molecules in various influenza viruses have different molecular structures of sialidase to crossly bind to different sialic acid receptors. The receptor of bird flu virus is sialic acid α2, 3-galactoside, while the receptor of human influenza virus is sialic acid α2, 6-galactoside. However, swine influenza virus has affinity to both receptors. In swine respiratory tract mucosa, both receptors have been found, indicating that swine is susceptible to both human and bird influenza viruses and swine flu virus can infect both human and birds. In addition, during the absorption and infection of influenza viruses, surface glycoprotein of the host cell membrane and other factors are also necessary. Human influenza prevalence occurred in Spain in 1918, the influenza epidemic in Asian in 1957 and the influenza outbreak in Hong Kong in 1968 are all closely related to the concurrent swine influenza outbreak.

6.2 Hereditary Variations of the Influenza A (H1N1) Virus

The quick variation of antigen is also characteristic of influenza virus. HA and NA are situated on the surface of virus envelope, being antigenic proteins with the highest variation rate. The antigenic variations of influenza viruses occur in the way of antigenic drift or antigenic shift. The antigenic variation by antigenic shift is great, usually giving rise to the emergence of new virus subtype.

7 Field Epidemiology

Field epidemiology is a study for investigation of and emergency responses to the outbreak of infectious diseases. It develops in emergency responses to serious and sudden public health events and it serves to solve unpredictable problems during prevalence of infectious diseases. Influenza A (H1N1) constitutes a major problem in the study of field epidemiology [2].

7.1 Influenza Surveillance System

Influenza surveillance system has already played and will continue to play an important role in emergency responses to Influenza A (H1N1) outbreak. According to the first five cases reports by CDC of the US, these patients visited the local clinics respectively in mid-April in San Diego and Imperial (California), Guadalope (Texas) with influenza-like symptoms and signs. These visits rendered them step into the routine influenza surveillance system of the US. In this system, local clinics and medical institutions the patients visited collected samples from the patients, followed by routine laboratory tests by local department of public health to screen Influenza A (H1N1) virus positive cases and by CDC of the US to determine its subtypes. Such an influenza surveillance system played an indispensable role in early laboratorily definitive identification of the pathogen in this pandemic of Influenza A (H1N1).

7.2 Public Health Professionals

Public health staffs including field epidemiologists definitely play the leading role in the emergency response to this outbreak of Influenza A (H1N1).

National Health Ministry of China timely released the breakthrough in Influenza A (H1N1) studies. Namely, a test reagent H for quick, simple and specific detection of Influenza A (H1N1) patients had been successful developed and produced. The test reagent H was delivered to equip the surveillance network labs of provinces and cities and solve key problems including port inspection and quarantine, screening of suspected cases, differential diagnosis from common influenza. The study of field epidemiology combines logical thinking in classic epidemiology, methods of modern descriptive and analytical epidemiology, and laboratory techniques, showing its advantages in emergency response to the outbreak of Influenza A (H1N1).

7.3 Positive Interaction and Extensive Collaboration

Positive interaction and extensive collaboration are two important measures in the prevention and control of Influenza A (H1N1). Public health emergency is not a internal responsibility of medical and health institutions, but a challenge to the whole society. After the outbreak of Influenza A (H1N1), WHO, once again, was the international organization and center coordinating the emergency responses of all countries in the world. In this system, close corporations were conducted between different areas/countries; health departments and social institutions constitute a government led joint forces; professional departments and disciplines concentrate superior forces to coordinate and cooperate with each other; the knowledge innovation and information update became much more scientific and sensible. This outbreak of Influenza A (H1N1) will be recorded in the history as a major sudden public health emergency and will also accelerate the further development of field epidemiology.