Currently, two main approaches are used in health care to control the spread of influenza: vaccination and the use of antiviral drugs. The influenza virus has the unique capability of being able to change its antigenic makeup; this mechanism, known as antigenic drift, occurs with all three types of influenza virus (A, B, and C). Influenza A shows the greatest rate of antigenic change. Antigenic drift is caused by sequential point mutations in the hem agglutination (HA) or NA genes that arise during viral ribonucleoprotein (RNP) replication and immune selection, giving rise to new strains; this gives the virus the ability to reinfect “nonimmune” susceptible hosts each season. Another phenomenon, antigenic shift, is manifested only by the influenza A virus. It involves complete reassortment of the segmented viral genome during a co-infection with a nonhuman animal, which results in major antigenic change and periodic worldwide out breaks (pandemics) of a never before circulated type of influenza A virus. Influenza B undergoes antigenic change very slowly.

Antigenic drift requires the reformulation of the influenza vaccine each year to ensure maximum efficacy against the currently circulating strains of influenza A and influenza B, because the vaccine is only as efficient as the influenza strains selected for it. This is accomplished by global surveillance of the yearly influenza epidemics to evaluate the strains that are circulating and provide early detection of viruses that may have pandemic potential. The World Health Organization (WHO) coordinates a influenza surveillance program in more than 80 countries. In the United States, the surveillance program established by the Centers for Disease Control and Prevention (CDC) includes monitoring of pneumonia and influenza deaths above a calculated “epidemic threshold.” It also includes tallying pediatric deaths, assessment of weekly virology data, and typing of influenza virus isolates submitted by reference laboratories. This extensive surveillance system provides the data for determining and predicting the influenza strains likely to be circulating in the upcoming winter, and vaccine components are chosen annually by WHO based on the analysis of these strains. The summer months are used to manufacture the vaccine so that it is ready for early autumn distribution to health care providers. In the United States, the vaccine is pre pared from viruses grown in embryonated chicken eggs; this is a trivalent vaccine containing two influenza A strains with the newest HA and NA surface antigens and a current type B strain.

Currently two types of vaccine are used to prevent influenza infection: the trivalent inactivated influenza vaccine (TIV) and the live attenuated influenza virus vaccine (LAIV). The TIV is a noninfectious vaccine administered intramuscularly. It currently is approved in the United States for individuals 6 months or older, including those with chronic medical conditions. It is 70% to 100% effective in preventing infection among healthy adults and 30% to 60% effective in the elderly and pediatric populations. The LAIV contains live whole infectious virus. It is administered intranasally and currently is approved in the United States for healthy individuals 2 to 49 years of age. Because it contains live virus, it is not recommended for immunocompromised individuals, the elderly, or people with reactive airway disease. The LAIV causes shedding of the virus that is detectable in rapid antigen assays for about a week.

Two classes of antiviral drugs, the adamantanes and the neuraminidase inhibitors, currently are used to treat influenza infections. The adamantanes, which include the drugs amantidine and rimantadine, were the first antiinfluenza class of antiviral treatment developed. Their mechanism of viral defense is blockage of the virion M2 ion channel, which prevents the virus from uncoating. This class of drugs is effective only at treating influenza A infections; it has never had any effect on influenza B infections. The neuraminidase inhibitors include the drugs zanamivir (Relenza) and oseltamivir (Tamiflu). Both of these drugs inhibit the viral protein neuraminidase, which prevents release of the virus from infected cells. The neuraminidase inhibitors are used to treat both influenza A and influenza B infections, although oseltamivir has been reported to have lower efficacy against influenza B. Both classes of drugs have proven to be most effective when administrated within 48 hours of symptoms. The drugs shorten the duration of the infection and reduce complications.

The need for effective influenza antiviral susceptibility surveillance has increased around the world, and its importance is validated by the emergence of universal resistance to the adamantine antiviral therapy for influenza A (H3N2). Samples of viruses collected from around the United States and worldwide are studied to deter mine whether they are resistant to any of the four influenza antiviral drugs approved by the U.S. Food and Drug Administration (FDA). The CDC, in collaboration with state public health departments and WHO, conducts ongoing surveillance and performs testing of influenza viruses to monitor for antiviral resistance. The number of surveillance sites, both domestically and globally, are being increased, and the data from this surveillance are used to make public health policy recommendations on the use of these antiviral medications. The CDC is constantly improving its methods of rapidly detecting and monitoring antiviral resistance. Laboratory methods for testing also are being improved, and the number of laboratories capable of testing for antiviral resistance is rising.

Antiviral resistance to the adamantanes among circulating influenza A (H3N2) viruses rapidly increased worldwide beginning in the 2003-2004 influenza season. Data from the CDC’s World Health Organization (WHO) Collaborating Center for Surveillance, Epidemiology and Control of Influenza reports that the percentage of influenza A (H3N2) virus isolates submitted from around the world that were adamantine resistant increased from 0.4% in the 1994-1995 season to 12.3% in the 2003-2004 season. This resistance continued to increase; during the 2005-2006 influenza season, the CDC reported that of 209 isolates, 193 (92%) of the influenza A (H3N2) iso lates carried a change at amino acid 31 in the M2 gene that confers resistance to the adamantanes. At the end of the 2008-2009 influenza season, 100% of influenza A H3N2, along with novel 2009 influenza A H1N1, were resistant to the adamantanes.

Resistance to oseltamivir appeared in the seasonal influenza A/H1N1 virus subtype during the 2007-2008 season. Oseltamivir resistance can result from a number 2007-2008 season, the CDC reported a nationwide resistance of 10.9% of the isolates submitted. This resistance also continued to increase; at the end of the 2008-2009 influenza season, the CDC reported that of 825 isolates of seasonal influenza A H1N1, 820 (99.4%) were resistant to oseltamivir. None of the other strains of influenza (i.e., influenza H3N2, novel 2009 influenza A H1N1, and influenza B) showed any resistance to the neuraminidase inhibitors (neither oseltamivir nor zanamivir).

For the 2010 influenza season, resistance to the adamantanes remained high; both circulating influenza A viruses (H3N2 and 2009 H1N1) showed high levels of resistance to the these drugs. These viruses are still susceptible to the neuraminidase inhibitors, and this class of antiviral medication is the current therapy of choice for antiviral treatment and for chemoprophylaxis of current circulating influenza A virus strains (Table 1).

Table1. Antiviral Agents

At the end of the 2009-2010 season, almost all (98.9%) of the 2009 H1N1 isolates characterized at the CDC were susceptible to oseltamivir (Tamiflu), and all (100%) were susceptible to oseltamivir (Relenza). The rare 2009 H1N1 oseltamivir-resistant influenza A viruses shared a single genetic mutation, causing them to be resistant to this antiviral medication. Many of the influenza A H5N1 strains (avian influenza) are resistant to the adamantanes, so oseltamivir is the current antiviral of choice. Early treatment with oseltamivir improves the chance of survival in individuals infected with this type of influenza virus, but the mortality rate for the disease remains high.

اخر الاخبار

اخبار العتبة العباسية المقدسة

المجمع العلمي يقيم محطة قرآنية على طريق الزائرين بذكرى شهادة الإمام الكاظم (عليه السلام) في واسط

قسم العلاقات العامّة ينظّم برنامجًا ثقافيًّا لأكثر من 100 طالب من حفظة القرآن الكريم في النجف

قسم شؤون المعارف يختتم دورة حول تحقيق المخطوطات في جامعة كربلاء

قسم الشؤون الطبية يطلق برنامجًا تدريبيًّا لملاكاته بالتعاون مع مستشفى الكفيل التخصصي

المزيد

الآخبار الصحية

6 فواكة تساعدك على الشعور بتحسّن عند المرض

حفاظا على صحة العين.. هذا أفضل وقت لتناول فيتامين أ

الاستخدام اليومي لسماعات الأذن.. أخطر مما تتصور !

هل توقيت نشاطك اليومي يؤثر على صحة دماغك؟.. العلماء يجيبون

المزيد

الآخبار التكنلوجية

ابتكار ألماني قد يغيّر مستقبل صناعة البلاستيك

تحديثات وأرباح عبر المشاهدات.. ما الذي تخطط له "لينكد إن"؟

"الكوكايين الوردي".. مخدر جديد يثير قلق السلطات الأميركية

دراسة: تغيّرات في أسلوب القيادة قد تكشف مبكرا عن الخرف !

المزيد

مواضيع ذات صلة

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Coronavirus Infections in Humans

Properties of Coronaviruses

Congenital Rubella Syndrome

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Hendra Virus and Nipah Virus Infections

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Retroviruses