Читать книгу Vaccines For Dummies - Sharon Perkins - Страница 22

Type A viruses cause us the most problems. They’re the only influenza viruses that have caused a global pandemic, like the COVID-19 pandemic, and they, along with type B, are the types that usually cause seasonal flu outbreaks. (See Chapter 3 for more about COVID-19.)

There are a lot of subtypes of influenza A. There may be 200 main subtypes out there, but only 131 main subtypes have been found. We can’t make a vaccine with that many different antigens — all those bits of the virus are needed to create a “Wanted” photo for each subtype. Usually, just a few subtypes dominate any flu season.

So, every year scientists have to make a decision — which two A subtypes and one, or likely two, B subtypes should go into a vaccine. The problem is that it takes a long time to make enough flu vaccines for everyone. Some types of the flu vaccine are grown in chicken eggs, and this takes time. (But don’t worry if you’re severely allergic to eggs; others are made just in the lab). It takes six months to make a vaccine, so scientists have to pick which subtypes to include half a year ahead of time. They peek at the other side of the globe (Northern or Southern Hemisphere) and figure what’s starting to spread, say, in Australia for Northern Hemisphere folks, and what they bet will spread where they are in six months.

Type A influenza viruses are unusual in that they have tricks to change what they wear, making those immune system “Wanted” photos not always work. They have more tricks for changing the proteins that are on their outside shell, and so they fool our immune system into not knowing who they are. Unlike other types of influenza viruses, Type A has two different ways to change and evade our immune systems. These two ways are called drift and shift. Influenza A can also infect some animals, and sometimes influenza of animal origins can be a problem for us:

 Drift, also called antigenic drift, happens when mutations occur over time. It occurs in all influenza types. After a few mutations, copies of the virus become a bit different from the original viruses. Over time, these small changes build up, and the virus may not even be recognized by our immune system. We can get sick from a subtype that’s similar to, but not the same as, one we’ve seen before. Vaccines continue to work as the virus mutates until there are enough mutations and the antibodies made in response to the vaccine won’t recognize the subtype anymore. These slight changes can lead to resistance to drugs we use to treat the flu or even change how this flu subtype affects us.

 Shift occurs only in type A influenza. Influenza A viruses each have two proteins on their surfaces: H (hemagglutinin) and N (neuraminidase). There are 18 different known H’s and 11 known N’s. That’s why there are so many different types of possible influenza A subtypes. These can mix and match — or shift — resulting in big changes in the flu subtype: H1N1, H1N2, H3N2, H7N9. These big changes can result in shifts that leave us unprepared for new subtypes and hence, the risk of flu pandemics.

The following sections go into more detail on two specific Type A viruses: bird flu and swine flu.