Lesson 7, Volume 15—Influenza: Recent Developments

By Robert L. Penn, MD

Objectives

1. To review the epidemiology of influenza.
2. To discuss the emergence of new influenza virus strains.
3. To discuss the clinical diagnosis of acute influenza.
4. To describe use of the influenza vaccine.
5. To review the options for anti-influenza drug therapy.

Key words

anti-influenza therapy; influenza; influenza vaccine; influenza virus

Abbreviations

CDC = Centers for Disease Control and Prevention; EIA = enzyme immunoassay; PCR = polymerase chain reaction

Virology

The influenza viruses are members of the Orthomyxoviridae family. They contain a segmented negative-sense RNA genome and include a lipid bilayer envelope. The three types, A, B, and C, have been categorized on the basis of antigenic and structural differences. Influenza A and B contain eight RNA gene segments, while type C contains seven segments. Types A and B cause most human infections and outbreaks, whereas type C infection is usually asymptomatic or a mild and sporadic illness.

Influenza A expresses two spike-like surface glycoproteins, the hemagglutinin (H) and neuraminidase (N) antigens. The hemagglutinin is a monomer (HA₀) that mediates attachment to cell-surface sialic acid. It is subsequently cleaved by host cell proteases in endosomes into two closely linked peptides (HA₁ and HA₂) that mediate virus-cell fusion.³ Antibodies to the hemagglutinin are an important determinant of host immunity. The neuraminidase (N) serves to cleave terminal sialic acid residues. It is a tetramer that has an active site that is conserved across viral types. This property has made it a useful target for new anti-influenza drug development. Potential roles for neuraminidase include the promotion of virion release from infected cells, prevention of viral aggregation, viral penetration of airway mucin, induction of cellular apoptosis through the activation of transforming growth factor (TGF-b), and the induction of cytokine release.⁴ Antibodies to neuraminidase are felt to contribute to limiting viral spread and thus limit the extent of infection.

The naming of influenza A and B virus strains follows a specified pattern that includes their type/origin/isolate number/year of isolation (hemagglutinin and neuraminidase subtype). For example, the trivalent vaccine for the winter of 2001 contains the strains as listed in Table 1.

Epidemiology

Influenza is maintained in humans by person-to-person spread. This occurs when virus from an infected individual comes in contact with the respiratory epithelium of susceptible hosts. The attack rate from such an exposure reflects viral virulence determinants, the amount of virus inoculated, and the new hosts’ level of immunity. The major route of spread is through small-particle aerosols generated by coughing, sneezing, and even talking. However, direct contact with infected secretions is also possible. The incubation period is relatively short, usually between 1 and 4 days, and this helps explain the explosive character of some outbreaks.

Epidemics of influenza almost always occur in the winter months, although the reasons for seasonal variation are unknown.¹ In the United States, influenza activity usually is greatest from December through March. Outbreaks are usually abrupt in onset, peak within 2 to 3 weeks, and may last for 6 to 12 weeks. Pediatricians are often the first to notice an influenza outbreak in their community as an increase in childhood febrile illness. Influenza-like illness then surfaces in adults, followed by an attendant increase in pulmonary and cardiac hospitalizations as complications of influenza in patients with these and other underlying medical conditions. The familiar Pneumonia and Influenza Mortality Rate reported by the Centers for Disease Control and Prevention (CDC) and local health departments is useful and specific, but is actually a lagging indicator of what already has been seen by practitioners in affected regions.

With intensified surveillance it has become apparent that influenza is often sporadically present year-round,⁵ and may result in early or summertime outbreaks.^6,7 When this occurs, the appropriate diagnosis may be delayed because influenza may not be in the initial differential diagnosis.⁶ Another source for summer outbreaks is the tourism industry. In July and August 1998, an influenza outbreak affected approximately 40,000 tourists on cruises and tourism workers in Alaska and the Yukon Territory, and a similar but smaller outbreak occurred in May and June 1999.^8,9 Official recommendations for regional travelers and tourism companies for prevention and control of such outbreaks emphasize the need to be aware of the risk, and the importance of rapid diagnostic tests and anti-influenza drugs.⁹

Small viral antigenic variations are felt to account in part for the annual recurrence of influenza, while more extensive antigenic changes help explain the occurrence of pandemics. Antigenic drift occurs in influenza virus types A and B, and represents point mutations within a subtype. It may involve either the hemagglutinin or the neuraminidase, but more often involves the hemagglutinin. Antigenic drift may appear rapidly and frequently, and the new strains quickly replace older strains to which the population has previously developed immunity. In contrast, antigenic shift represents the emergence of an immunologically distinct virus to which there is no immunity. This occurs at less frequent and irregular intervals in influenza virus type A, and represents a more dramatic change in the hemagglutinin only or both the hemagglutinin and the neuraminidase. Antigenic shift is the consequence of genetic reassortment in animals or the direct transmission of an animal strain to humans.

Emergence of New Viruses

Aquatic birds are the reservoirs for influenza A in nature. Wild avian hosts are usually asymptomatic, and infection is confined to their GI and/or respiratory tracts. The emergence of new human influenza A viruses is favored by their inherent variability. Polymerase errors may result in point mutations that may alter important viral structures. In addition, the segmented nature of the genome facilitates genetic reassortment, and a large number of strains are possible given the 15 hemagglutinins and nine neuraminidases that have been identified among avian influenza viruses. Nonetheless, most recent human infections have involved only three hemagglutinins (H1, H2, H3) and two neuraminidases (N1, N2).

In general the host range for any specific virus is limited so that, for example, avian viruses do not multiply well in humans and human viruses do not multiply well in avian species. Receptor specificity is an important mechanism acting to restrict influenza viruses to a specific host. The hemagglutinin of avian viruses binds preferentially to a2-3 galactose linkages on their sialyloligosaccharides, while the hemagglutinin of mammalian viruses prefers a2-6 galactose linkages on their sialyloligosaccharides.¹⁰ Pigs can be infected with both avian and mammalian viruses, however, because swine tracheas contain both a2-3 and a2-6 galactose linkages.¹⁰ Thus, they are likely the natural "mixing vessels" for genetic reassortment between avian and human strains. This is supported by the observation that pigs and birds are often in close proximity in areas of the Far East where new influenza viruses emerge.

An influenza virus with pandemic potential must not only demonstrate antigenic shift, it must possess other as yet undefined genetic traits that confer virulence in humans. Recent investigations have focused on sequencing the 1918 pandemic virus in an attempt to better understand its virulence.¹¹ Lung tissue was obtained from three victims of the 1918 epidemic who died rapidly with a clinical picture of viral pneumonia. Two were paraffin specimens from autopsies archived at the Armed Forces Institute of Pathology. The other was tissue obtained from an Inuit woman preserved by permafrost in a mass grave at Brevig Mission, Alaska. This village was decimated by the 1918 influenza over just a 5-day period, with 72 deaths that included 85% of its adult population. It proved possible to amplify fragments of the viral RNA genome from these specimens. Sequencing overlapping segments elucidated the complete gene sequences encoding the hemagglutinin and neuraminidase, and sequencing of other genes is in progress.¹¹ This virus was an H1N1 influenza A strain, very closely related to the swine strain A/Sw/Iowa/30. However, both the hemagglutinin and the neuraminidase share many avian features. Unfortunately, no genetic changes have been found as yet that are known to affect virulence. Phylogenetic analysis of the data suggests that the 1918 influenza virus was very close to a common mammalian ancestor virus. Together, these results are compatible with an avian virus that first adapted in a mammalian host a short time prior to emerging as the 1918 pandemic strain, or alternatively an avian virus that directly entered humans to cause the 1918 pandemic. Thus, the origin of the 1918 virus and any special virulence properties it may have possessed are still uncertain.¹¹

Avian Influenza

Influenza viruses of avian origin recently have demonstrated their ability to directly infect humans. In 1997, a total of 18 cases of influenza caused by A/Hong Kong/156/97 (H5N1) occurred in Hong Kong. This was associated with an outbreak of severe disease in chickens caused by a genetically similar virus that also had multibasic insertions in the hemagglutinin cleavage site. This increased the tissue tropism of the virus in chickens by permitting hemagglutinin cleavage by ubiquitous proteases; however, it is not known if this was a factor in the human illness. Illness was often most severe in adults, and 6 of the 18 cases were fatal.¹² No new cases occurred after the live poultry population of Hong Kong was slaughtered. Viral genes were of avian origin and secondary cases were not detected, supporting direct spread from poultry to humans. Recent serologic studies have found evidence of infection among some close contacts of these cases, suggesting that human-to-human spread may have occurred.^12,13 The exact mechanisms underlying the failure of this virus to cause more widespread disease are uncertain, but inefficient spread from person to person certainly contributed.

Intensive surveillance in Hong Kong of poultry viruses detected influenza A (H9N2) also present in 1997. Two years later, in 1999, H9N2 influenza was diagnosed in two children living in different parts of Hong Kong.¹⁴ Their viruses were similar to a poultry virus isolated in 1997, but the internal genes were similar to the H5N1 virus isolated from humans in 1997.¹⁵ These observations underscore the rapid changes that are possible in new influenza viruses, the potential for avian viruses to infect humans, and the need for ongoing surveillance.

Clinical Diagnosis

Influenza often presents with characteristic signs and symptoms. Typical influenza-like illness starts suddenly with the onset of myalgias, arthralgias, fever, chills, and headache. Minor sore throat and cough progress over time to become more prominent as the systemic symptoms abate, typically after the third day. Early in influenza, the physical examination may reveal only fever and mild conjunctival injection. When present, particularly in an outbreak setting, a presumptive clinical diagnosis may be made on the basis of such a typical clinical picture. However, diagnostic studies are still warranted to plan antiviral therapy and to monitor circulating strains.

Recent observations have emphasized the importance of influenza as predisposition for certain clinical presentations. These include otitis media and severe pneumococcal pneumonia in children, hepatic decompensation in cirrhotic or liver transplantation patients, and increased cardiopulmonary hospitalizations during pregnancy.^1,5,16-18

The gold standard for laboratory confirmation of influenza is viral isolation. Appropriate specimens include nasal washes, nasal and throat swabs together, sputum, and bronchoscopy material. Throat swabs alone are less sensitive than the others.¹ Although it may take 1 week for a specimen to be positive, the majority of isolates are detected within 3 days of incubation.¹ Centrifugation onto shell vial cultures usually shortens the time to virus detection.

Rapid diagnosis is of more immediate value to the clinician and the patient. For example, it recently was demonstrated that rapid diagnosis of influenza A in a pediatric emergency room decreased antibiotic use and increased anti-influenza drug therapy.¹⁹ Several viral antigen detection kits are commercially available that can provide results in less than an hour.^5,20 Methodologies include enzyme immunoassay (EIA), optical immunoassay, and detection of neuraminidase activity. Indirect immunofluorescence (IFA) requires more technical skill and is more labor-intensive, taking several hours to yield results. Immunoassays can distinguish between influenza A and B. However, neuraminidase detection alone cannot make this distinction. When compared with viral isolation, such antigen detection tests have had a sensitivity of 54 to 96% and a specificity of 92 to 99%; the performance has varied with the setting and with the type and quality of specimens processed.^5,21,22 A neuraminidase detection assay (Zstat Flu) was directly compared with immunofluorescence, EIA, and viral culture for detecting influenza in nasal washes performed by respiratory therapists at a children’s hospital.²¹ Viral detection by all methods in general related to the cellularity of the specimen. In this study, the neuraminidase detection assay had a sensitivity of 76.4% for influenza A and 40.9% for influenza B, while the EIA test (that could detect only influenza A) had a sensitivity of 89.7% for influenza A.²¹

Thus, clinicians should be aware that false-positive and false-negative results occur with these tests. Sending the best specimen possible in the appropriate transport media can optimize test performance. Also, it is important to be aware of which test your laboratory is using; some rapid antigen detection tests do not detect influenza B viruses and should not be relied upon alone when influenza B is circulating. Other tests may detect both influenza A and B but do not distinguish between them.²⁴

Serology to detect anti-influenza antibodies is limited by the fact that it requires a four-fold or greater rise in titers between acute and convalescent samples taken 10 to 21 days apart. Thus, serologic diagnosis is rarely useful in the management of the acutely ill patient. However, it can be helpful to confirm a diagnosis of influenza that had not been established by other means.

Influenza virus RNA detection using the polymerase chain reaction (PCR) is a promising technology for the diagnosis of influenza. PCR can detect nonviable virus, and care must be taken to avoid laboratory contamination.⁵ In a study of PCR for epidemiologic monitoring in Scotland, results were available within 36 h, somewhat longer than for the antigen detection tests discussed above.²³ One interesting approach is to combine PCR assays for multiple respiratory pathogens into a single panel termed multiplex PCR, offering potentially important information for clinical management while cultures and other diagnostic studies are pending. However, the most appropriate use of PCR methods and the optimum primers have not been determined.

Influenza Vaccine

Annual influenza vaccination remains the best means for the prevention of influenza. Vaccination is 70 to 90% effective in preventing influenza illness in healthy children and adults < 65 years of age when the vaccine strains are similar to the circulating strains.²⁴ In these circumstances vaccine also decreases days off work and use of health-care resources.²⁴ In children, extra benefits of vaccination include reduced incidence of otitis media and a reduced use of antibiotics.²⁴

The elderly have attenuated antibody responses to influenza vaccine so that vaccine efficacy in preventing influenza in this population is lower, ranging from 30 to 40%. However, vaccination of the elderly living in the community offers better protection against severe illness and influenza complications, and is cost-effective.²⁵ Vaccination of nursing home residents is 50 to 60% effective in preventing pneumonia and hospitalization, and 80% effective in preventing death.²⁴ Vaccine is also beneficial for the elderly with chronic lung disease; in a recent study, vaccination reduced pneumonia and influenza hospitalizations by 52% and mortality by 57%, and also significantly reduced outpatient visits for influenza and respiratory illnesses.²⁶

Table 1 lists the components of the 2001–2002 vaccine. The vaccine is again a killed preparation that is initially grown in eggs. The only general contraindication to vaccination is known anaphylactic hypersensitivity to eggs or other vaccine components. Additives vary by manufacturer, so the package insert should be consulted for specific recommendations. Modern vaccines do not increase the risk of Guillain-Barré syndrome.^24,27 Table 2 lists the target groups cited by the CDC for this year’s vaccine.²⁴ New last year and continuing this year is the recommendation to vaccinate all persons aged þ 50 years, instead of the previous target of þ 65 years.

Manufacturing problems with the 2000-2001 vaccine, including lower than expected yields, resulted in delayed vaccine availability and a reduced supply in some localities. Because of this the CDC issued adjunct recommendations for the 2000-2001 season.^24,28 They included proceeding with vaccination of high-risk persons and health-care providers as originally planned, and delaying organized influenza vaccination campaigns. In particular, the general vaccination of all persons age 50 to 64 was to be delayed until December and later.²⁴ Health-care providers were to continue to routinely offer vaccine from its initial availability to after November 2000 despite the fact that the usual target season is September through November.

Delivery delays and shortages remain possible again this season and in the future. Since influenza activity has peaked recently in the months of December through March, vaccine may be given after November and should be beneficial.²⁴

Live, attenuated, intranasal influenza vaccines offer great promise for improving vaccine acceptability and efficacy. These vaccines are in use in Russia, and have undergone development in the United States for more than 20 years.²⁴ Donor influenza A and B strains have been cold-adapted so that they grow well at low temperatures but not at core temperatures, and have been attenuated. Such cold-adapted viruses are then used to create reassortant strains to which the chosen hemagglutinin and neuraminidase subtypes have been added. This results in live, attenuated, cold-adapted viruses of the needed hemagglutinin and neuraminidase types for use as a vaccine that will grow in the upper respiratory tract but cause few symptoms. Belshe et al²⁹ reported on their randomized, double-blind, placebo-controlled trial of one or two doses of an intranasal vaccine (FluMist; Aviron; Mountain View, CA) in 1,070 healthy children aged 15 to 71 months. Vaccine was 93% effective in preventing confirmed influenza, and reduced febrile otitis media by 30%.²⁹ Results from a second year of this study were recently published.³⁰ During this follow-up season, the vaccine A (H3N2) component was a poor match for the predominant circulating strains, which were influenza A/Sydney/5/97 (H3N2)-like. Nonetheless, the intranasal vaccine was 86% effective in preventing confirmed A/Sydney influenza in children.³⁰ During this same season, a randomized, double-blind, placebo-controlled trial of the intranasal vaccine was undertaken in 3,041 healthy working adults aged 18 to 64 years.³¹ In this group, 70% of the vaccine doses were self-administered, and vaccine reduced the numbers of febrile illnesses by 13 to 27%, febrile respiratory illnesses by 9 to 24%, absenteeism by 18 to 28%, provider visits by 25 to 41%, and antibiotic use by 28 to 47%.³¹ Side effects attributable to the intranasal vaccine were limited and consisted of minor sore throat and runny nose.³¹ By themselves, these vaccines are not as immunogenic in the elderly. However, intranasal live vaccine combined with standard inactivated vaccine over three seasons reduced documented influenza by 61% among 523 nursing home residents, when compared with inactivated vaccine alone.³² The safety of trivalent intranasal vaccine (FluMist) with inactivated vaccine was evaluated in 200 health-maintenance organization members > 64 years of age and with underlying lung or heart disease or diabetes.³³ During the 28 days after vaccination, sore throat in the first week was the only side effect of the intranasal vaccine.³³

Potential advantages of the live virus intranasal vaccine are its acceptability, ease of use, and the induction of mucosal immunity. In addition, these vaccines may elicit a broader immune response than the inactivated vaccine and cross-protect against more antigenic variant strains in a fashion similar to natural influenza. However, some potential concerns remain that are beginning to be addressed. These include the safety of the vaccine for immunosuppressed patients, potential reversion to wild-type, and the immunogenicity of each season’s reassortant viruses.³⁴ Preliminary results show the live intranasal vaccine to be safe in HIV-infected persons³⁵ and to be genetically stable.³⁶ It is anticipated that a live virus intranasal vaccine will be commercially licensed in the near future.

Anti-Influenza Drug Therapy

Treatment of uncomplicated influenza has been enhanced with the recent availability of two new agents. This currently offers clinicians and health-care groups the option of four different antiviral drugs for influenza as listed in Table 3 and Table 4.²⁴ Choosing among them involves understanding their spectrum of activity, potential side effects, and costs.

Amantadine and rimantadine are older oral agents active against only influenza A (Table 3). They act by inhibiting viral protein M2 so that uncoating of virus is impaired.⁵ Both must be given within 48 h of onset of illness, and are equally effective as treatment in otherwise healthy adults and children (although rimantadine was approved only for prophylaxis in children).²⁴ Small airways dysfunction can be documented in uncomplicated influenza, and amantadine treatment is associated with a more rapid resolution of this abnormality compared with placebo.¹ Their efficacy in high-risk persons and in more complicated influenza has not been well studied. Amantadine and rimantadine are 70 to 90% effective as prophylaxis for preventing influenza A illness.²⁴

Amantadine may have prominent CNS side effects, particularly in the elderly and persons with renal insufficiency. Rimantadine is less likely to cause CNS toxicity, and is better tolerated in the elderly.³⁷ Both drugs may be associated with seizures in patients with underlying seizure disorders, and should be used with caution in such individuals. Concomitant use of amantadine with CNS stimulants, antihistamines, and anticholinergic agents should be avoided.²⁴ Reduced dosages are indicated to limit toxicity in the clinical circumstances listed in Table 3 and in children. Further information is available in the product inserts and in annual updates published by the CDC.²⁴ Resistant virus may emerge during treatment with either agent, so therapy should be stopped 1 to 2 days after resolution of signs and symptoms (usually 3 to 5 days total). Cross-resistance between amantadine and rimantadine is the rule. Their use in pregnancy has not been evaluated, and high doses of both drugs are teratogenic in animal models.²⁴

Two neuraminidase inhibitors, zanamivir and oseltamivir, have become available that offer some advantages for influenza therapy (Table 4).⁴ Importantly, unlike the older agents, they are active against both influenza A and B viruses and are approved for treatment of influenza A and B. Treatment must begin within 2 days of illness, but is not indicated for minor, nonfebrile illness in immunocompetent patients.⁴ Clinical trials found that illness lasted 1 to 2.5 days less with zanamivir, with similar efficacy for influenza A and B.⁴ Oseltamivir clinical trials found that illness lasted 1.2 to 1.5 days less with treatment compared with placebo.⁴ A recent randomized, controlled trial found that oseltamivir shortened uncomplicated confirmed influenza illness by 25 to 30% (approximately 2 days) when compared with placebo in patients treated within 36 h of symptom onset.³⁸ Zanamivir is 84% effective and oseltamivir is 82% effective in the prevention of influenza illness, although neither agent is as yet approved for prophylaxis of influenza.²⁴ As with the older agents, the efficacy of the neuraminidase inhibitors for high-risk persons and in more complicated influenza has not been well studied. Resistance to these agents has been infrequent.

Zanamivir is poorly absorbed when given by mouth so it is inhaled as a dry-powder aerosol using a plastic device supplied with the prescription. Only approximately 15% of the inhaled dose is systemically absorbed.⁴ Oseltamivir is available in capsule form. Neither drug has significant CNS toxicity, and reduced dosages are not required for the elderly. Zanamivir dosing does not need to be adjusted for renal insufficiency. In contrast, oseltamivir dosing must be reduced if the creatinine clearance is ú 30 mL/min, but there are limited data about its use in dialysis patients. No information is available at present about the use of these agents in patients with significant hepatic dysfunction. The neuraminidase inhibitors have not been evaluated for use in pregnancy, although zanamivir may offer a theoretical advantage because of its low systemic absorption. Neither drug is known to be teratogenic in animals.⁴ No significant drug interactions have been reported with zanamivir. Since oseltamivir is eliminated in part by renal tubular secretion, probenecid can increase its plasma levels.⁴ Further information is available in the product inserts and in annual updates published by the CDC.²⁴

Most side effects with inhaled zanamivir have been equal to those with placebo. However, during zanamivir treatment broncospasm and deteriorating respiratory function may occur. Some patients with asthma or underlying COPD have experienced a >20% drop in FEV1 or peak flows, or respiratory distress.^24,39 The Food and Drug Administration has advised that zanamivir is generally not recommended for patients with underlying airways disease because of the risk for bronchospasm and because efficacy in this population has not been demenstrated. Those patients with asthma or underlying COPD who use zanamivir should have a rapid acting inhaled bronchodilator available, and should stop zanamivir and contact their physician if they experience breathing difficulties.²⁴ Oseltamivir has had relatively few side effects reported to date. Its major adverse effects are gastrointestinal; nausea occurs in 10% and vomiting in 9%.²⁴ These symptoms may be reduced by administering oseltamivir with food.

Optimum use of these agents requires a rapid diagnosis of influenza.⁴⁰ During outbreaks of confirmed influenza a clinical diagnosis is sufficient. However, sporadic cases will benefit from the use of rapid diagnostic testing as discussed above. If influenza B is circulating or confirmed, then a neuraminidase inhibitor should be chosen. If influenza A infection is confirmed, then the choice can be made on the basis of toxicity profile and cost.⁴¹ Amantadine is significantly less costly than rimantadine, which is less expensive than the neuraminidase inhibitors.⁴¹

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