Rate-Limiting Steps in the Seasonal Vaccine Process
One of the most time-consuming steps in the seasonal process is the selection of updated reference strains each year. Delays are often justified because the epidemiology or transmission of a particular strain of influenza can change during the course of a flu season, and it is crucial that the best match for all three virus strains be included in the vaccine. Sometimes, one of the strains is not available as an egg-derived isolate. Current licensing requirements stipulate that isolates be passed exclusively in embryonated chicken eggs or cell cultures derived from embryonated chicken cells. If that strain is missing from the vaccine and is widespread during the flu season, the “mismatch” of vaccine and virus can substantially decrease the effectiveness of the vaccine.

A considerable amount of time is spent on steps beyond the manufacturer’s control, such as the creation of reassortants through the WHO Global Influenza Program. Furthermore, the collaborative, highly regulated process of standardizing the antiserum for each strain takes about eight weeks. In addition, a good deal of time is devoted to product-quality assurance or conformance to finished product requirements, as regulatory groups confirm potency values reported by the manufacturer.

Unique Aspects of Manufacturing Influenza Vaccine
The manufacture of influenza vaccine, although considered conventional, is unique in a number of ways. First, it is a global enterprise. The vaccine manufacturer is just one participant in a consortium of regulatory agencies, surveillance laboratories, certified laboratories, hospitals, and clinicians, as well as providers of embryonated eggs, components, equipment, raw materials, and transport agencies.

Second, the composition of the trivalent vaccine changes almost yearly. The vaccine must be licensed yearly (for every amendment), and the license is granted for one year only (July 1 to June 30). This has significant implications for the manufacturer in planning a production campaign:

• Unused formulated and filled vaccine is usually discarded.
• Monovalent concentrates are not typically used after 12 months from pool.
• Yields of the strains are variable and are not known by the manufacturer until commercial scale-up production.

Third, formulations are updated twice a year to reflect the epidemiologies in the northern and southern hemispheres. The manufacturer thus has a very short window of opportunity to respond to changes. If the release of reassortants is delayed because of epidemiological changes during the course of the flu season, this window can be very, very short. The currently licensed influenza vaccine is trivalent; thus if one of the strains cannot be produced, there is no vaccine. Balancing production schedules to provide vaccine in the northern hemisphere for the October 1 immunization campaign can be challenging.

Producing Vaccine for a Pandemic
Seasonal and pandemic influenza vaccines are clearly interrelated. The capacity available for the production of a pandemic influenza vaccine is largely based on current seasonal capacity. Even though a pandemic vaccine is expected to be a monovalent formulation (e.g., H5N1 or H7N7), it will still require using existing processes and capacity because there will be no time either to develop new processes or to expand capacity.

Some 300 million doses of trivalent vaccine are available worldwide. Theoretically, assuming a monovalent pandemic vaccine, 900 million doses could be produced worldwide. However, manufacturing data indicate that the number of doses per egg would be fewer than during typical seasonal production. Clinical data reported on experience to date indicate that significant amounts of antigen, more than for seasonal vaccines, will be necessary to protect even healthy young adults.

Stockpiling is a key element of pandemic preparedness. However, the strain(s) in stockpile vaccines for possible pandemic influenza must be continually updated. Emerging epidemiology indicates that there are two distinct clades in the H5N1 influenza virus (a clade change is indicated when there is a change in the HA gene tree phylogeny). In other words, the ancestral relationship among H5 hemagglutinin (HA) genes from H5N1 avian influenza viruses collected in a specified region has changed (WHO Global Influenza Program Surveillance Network, 2005)

Seasonal influenza vaccination rates are still too low (MMWR, 2001). In the U.S. population, vaccination rates for healthy young adults and children are significantly below the 90 percent target. Figure 7 shows that the vaccination rates for people aged 65 and older are higher but are still below 90 percent. Therefore, meeting the 2010 target rate of 90 percent for each age group might provide an incentive and rationale for expanding manufacturing capacities. This expanded capacity might then be available to meet the demand in the event of a pandemic.

Keep in mind that capacity expansion is a time-consuming process and should not be considered a rapid solution for the vaccine industry. At best, expansion will take four to five years from concept design to validation and licensure.

Engineering Opportunities
Egg-based vaccine technology has been used to produce seasonal vaccine for more than 30 years. Recently, egg-based vaccine technology has also been used to produce more than 30 million conventional doses of H5N1 vaccine and pilot lots of H7N7 vaccine. The technology has been steadily improving since it was introduced and is becoming increasingly automated. However, technological improvements in alternate release and quality assurance assays would be beneficial. In addition, increasing immunization rates could lead to an expansion of manufacturing capacity.

Cell-culture technology is likely to supplement egg-based technology. However, cell-culture technology will have to be very robust and will still focus on the same physical parameters for vaccine purification, extraction, and HA enrichment. In the near term, constraints related to the availability of strains and potency reagents will present constraints for both egg-based and cell-culture technologies.

References
DHHS (Department of Health and Human Services) and Centers for Disease Control and Prevention (CDC). 2006. Available on line at: http://www.cdc.gov/flu/weekly/weeklyarchives2002-2003/02-03summary.htm and http://www.cdc.gov/flu/weekly/weeklyarchives2003-2004/03-04summary.htm.
Ludwig S., O. Planz, S. Pleschka, and T. Wolff. 2003. Influenza-virus-induced signaling cascades: targets for antiviral therapy? Trends in Molecular Medicine 9(2): 46–52.
MMWR (Morbidity and Mortality Weekly Report). 2001. Influenza and pneumococcal vaccination levels among persons aged > or = 65 years—United States, 1999. National Health Interview Survey (NHIS). MMWR 50(25): 532–537 (’01,’03, Jan-Jun ’04).
Reichelderfer, P.S., A.P. Kendal, K.F. Shortridge, and A. Hampson. 1989. Influenza Surveillance in the Pacific Basin; Seasonality of Virus Occurrence: A Preliminary Report. Pp. 412–444 in Current Topics in Medical Virology, edited by Y.C. Chan, S. Doraisingham, and A.E. Ling. Singapore: World Scientific.
WHO Global Influenza Program Surveillance Network. 2005. Evolution of H5N1 avian influenza viruses in Asia. Emerging Infectious Diseases 11(10). Available on line at: http://www.cdc.gov/ncidod/EID/vol11no10/05-0644.htm.