Although this is the current leading strategy, it is a highly complex and time-consuming process, typically taking 6–8 months to produce sufficient quantities. Standard flu vaccines are formed from inactivated and liveattenuated viruses produced using chicken eggs – every year, vaccine producers consume millions of eggs in this process. ![]() As a result, vaccine formulations must be regularly updated to keep up with emerging forms of the flu viruses. The 1918 Spanish Flu and 1968/1969 flu pandemic are two notable historical examples of this. These drastic changes allow influenza viruses to catch our immune systems completely off-guard and rapidly spread through populations, leading to pandemics. Antigen shift, on the other hand, is much rarer and is defined by abrupt changes to HA and NA through genome reassortment by influenza A with other influenza subtypesthat are co-infecting the same host cell, made possible by the segmented nature of the influenza virus genome. These changes accumulate over time, transforming the viruses and allowing them to escape host recognition without hindering their ability to gain entry to cells. Antigenic drift occurs when influenza viruses continuously undergo changes to their HA and NA surface proteins through mutation. This poses a challenge for vaccine formulation because evolutionary changes in these proteins enable the virus to evade the adaptive immune response through a combination of phenomena known as antigenic drift and shift (1), as shown in Figure 1. ![]() ![]() Most influenza vaccines elicit antibodies against the major viral surface proteins, hemagglutinin (HA) and neuraminidase (NA). Antigenic drift is the gradual accumulation of smaller mutations, whereas antigenic shift is a sudden, drastic change.
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