Influenza A (H1N1): A Science-Based Guide to Prevention and Control

 

 

Influenza A (H1N1) is an acute respiratory illness caused by a novel influenza A (H1N1) virus. Unlike typical Seasonal influenza viruses, this novel strain is a hybrid containing genetic material from swine, avian, and human influenza viruses. The virus is highly contagious, and is spread easily between people. In 2009, China listed Influenza A (H1N1) as a Category B notifiable disease while implementing the stringent prevention and control measures normally reserved for Category A diseases. It was subsequently downgraded to Category C in 2013.

I. Epidemiological Profile

The infection is transmitted by humans or animals carrying the influenza A (H1N1) virus, including asymptomatic individuals. So far, no evidence of animal-to-human spread has been found. The primary mode of spread is through respiratory droplets. It can also spread via contact with the mucous membranes of the mouth, nose, or eyes, either directly or indirectly. Infection may also occur through contact with infectious materials, including respiratory secretions and other body fluids from patients, or with contaminated environmental surfaces.

While the disease can affect all age groups, most cases are reported in adults aged 25 to 45, predominantly young and middle-aged individuals. Among those at highest risk for severe illness are laboratory personnel exposed to the influenza A (H1N1) virus or infectious samples as part of their work, pregnant women, individuals with pre-existing chronic conditions, patients under 19 on long-term aspirin therapy, individuals with obesity, children under 5, and adults over 65. Infections with Influenza A (H1N1) in humans show a clear seasonal pattern, peaking in winter and spring.

II. Clinical Presentation

The incubation period of Influenza A (H1N1) is approximately 1 to 7 days, typically 1 - 3 days. Most patients present with classic influenza-like symptoms, such as fever, sore throat, runny nose, cough, headache, generalized muscle aches, chills, and fatigue. Gastrointestinal symptoms such as vomiting and diarrhea, as well as muscle pain and conjunctival redness, may also occur. In a minority of cases, illness is limited to mild upper respiratory symptoms without fever.

Clinical signs primarily include pharyngeal hyperemia and tonsillar hypertrophy, while pulmonary findings are often minimal. Some patients, however, may present with moist rales or signs indicative of lung consolidation. In severe cases, the disease can deteriorate abruptly, marked by sudden high fever exceeding 39°C. This may progress to serious complications such as severe pneumonia, acute respiratory distress syndrome (ARDS), pulmonary hemorrhage, pleural effusion, pancytopenia, renal failure, sepsis, and shock. The disease may even culminate in critical conditions such asReye syndrome, respiratory failure, multi‑organ injury, potentially leading to death. Additionally, any pre‑existing medical conditions are likely to worsen in this setting.

III. Laboratory Findings

(I) Etiological Tests. A positive result for Influenza A (H1N1) is confirmed by detecting viral RNA via Reverse transcription polymerase chain reaction (RT-PCR) testing in a respiratory specimen, such as throat swab, nasal swab, nasopharyngeal or tracheal aspirate, and sputum. In addition, the Influenza A (H1N1) virus may also be isolated from respiratory specimens through cell culture. Furthermore, a fourfold or greater rise in virus-specific antibody titers between acute- and convalescent-phase serum samples confirms recent infection.

(II) Complete Blood Count (CBC). The total white blood cell count is typically within the normal range or decreased.

(III) Blood Biochemistry. While some patients may present with hypokalemia, others may show elevated levels of creatine kinase (CK), aspartate transaminase (AST), alanine aminotransferase (ALT), and lactate dehydrogenase (LD).

IV. Treatment Strategies

Supportive Care: Patients are advised to ensure ample rest, maintain adequate fluid intake, and closely monitor their clinical condition. Antipyretic medication may be used to manage high fever and improve comfort.

Antiviral Therapy: The Influenza A(H1N1) virus is susceptible to neuraminidase inhibitors (NAIs), includingOseltamivir and Zanamivir. In addition, certain traditional Chinese patent medicines, such as ShufengJiedu Capsules and Lianhua Qingwen Capsules, have also demonstrated therapeutic benefit in clinical practice.For high-risk individuals prone to severe disease, antiviral therapy is recommended to be initiated as early as possible upon the appearance of influenza-like symptoms.

V. Port-of-Entry Screening and Control

Identification of Suspected Cases. At points of entry, a determination of suspected Influenza A (H1N1) infection is made based on an integrated assessment of the traveler’s epidemiological risk (travel and contact history), symptoms, and physical signs. The first step is to assess travel history, which involves determining if the person has come from or passed through an Influenza A (H1N1)-affected area in the preceding seven days, with reference to daily global outbreak information. The second step is to establish contact history. This requires determining whether the traveler has had: contact with a confirmed Influenza A (H1N1) patient or their bodily fluids and excretions; contact with any individual exhibiting acute respiratory symptoms; contact with surfaces or objects potentially contaminated with the virus; visited crowded or poorly ventilated public spaces, hospitals, or clinics; taken any medications; or got the seasonal influenza vaccine. The third and final step is a clinical assessment of symptoms and signs. This evaluation involves checking for the presence of key symptoms - including fever, fatigue, headache, generalized muscle aches, runny nose, as well as cough - and conducting a physical examination to identify nonexudative pharyngitis and note any occasional rales on chest auscultation.

Auxiliary Testing. Following the identification of a suspected case, on-site collection of biological specimens - such as throat swabs, nasal swabs, throat wash samples, and blood - is initiated for rapid testing. These samples are then forwarded to a certified laboratory for definitive etiological confirmation. Informed consent must be obtained from the individual and documented by a signed Informed consent for sampling  before any samples are collected.

Management of Close Contacts. Once a traveler is classified as a suspected case of Influenza A (H1N1), individuals identified as close contacts are subject to the same comprehensive medical screening protocol detailed above. Asymptomatic close contacts may be permitted to proceed after completing three key steps: registration of their personal information, receipt of specific health guidance, and issuance of a Medical Convenience Card  to expedite future healthcare access if needed.

 

 

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