The high variability and rapid evolution of influenza virus antigens, however, is a major obstacle in developing effective vaccines. Furthermore, it is hard to predict which strains will be in circulation during the next flu season, manufacturing a sufficient quantity of flu vaccines for the next season is difficult, LAIVs have limited efficacy, and repeated annual vaccination potentially has diminished efficacy. For these reasons, "broadly-reactive" or "universal" flu vaccines are being researched that can provide protection against many or all influenza viruses. Approaches to develop such a vaccine include HA stalk-based methods such as chimeras that have the same stalk but different heads, HA head-based methods such as computationally optimized broadly neutralizing antigens, anti-idiotypic antibodies, and vaccines to elicit immune responses to highly conserved viral proteins. mRNA vaccines to provide protection against influenza are also under research. In animals
Birds
Aquatic birds such as ducks, geese, shorebirds, and gulls are the primary reservoir of IAVs. In birds, AIVs may be either low pathogenic avian influenza (LPAI) viruses that produce little to no symptoms or highly pathogenic avian influenza (HPAI) viruses that cause severe illness. Symptoms of HPAI infection include lack of energy and appetite, decreased egg production, soft-shelled or misshapen eggs, swelling of the head, comb, wattles, and hocks, purple discoloration of wattles, combs, and legs, nasal discharge, coughing, sneezing, incoordination, and diarrhea. Birds infected with an HPAI virus may also die suddenly without any signs of infection.The distinction between LPAI and HPAI can generally be made based on how lethal an AIV is to chickens.
Metirosine (INN and BAN; α-Methyltyrosine, Metyrosine USAN, AMPT) is an antihypertensive drug. It inhibits the enzyme tyrosine hydroxylase and, therefore, catecholamine synthesis, which, as a consequence, depletes the levels of the catecholamines dopamine, adrenaline and noradrenaline in the body. It is available as a generic medication. Clinical use
Metirosine has been shown to suppress catecholamine synthesis and alleviate symptoms related to catecholamine excess, including hypertension, headache, tachycardia, constipation, and tremor. Metirosine is primarily used to reduce these symptoms in patients with pheochromocytoma. It is contraindicated for the treatment of essential hypertension. Metirosine is used as an off-label treatment for DiGeorge syndrome.Metirosine is used in scientific research to investigate the effects of catecholamine depletion on behavior. There is evidence that catecholamine depletion causes an increase in sleepiness that is more pronounced than sleep deprivation, and that the fatigue lingers after the drug is discontinued. Negative mood is also a reported side effect of catecholamine depletion, although this is reported less consistently than sleepiness. See also
p-Chlorophenylalanine

References
External links
"Metyrosine". Drug Information Portal. U.S. National Library of Medicine.