affect many animal species. Some flu viruses just remain in other animals, but some flu subtypes can spread to us from animals, especially farm animals like chickens and pigs. Science has created influenza vaccines against some but not all types. The yearly flu vaccine consists of antigens against type A and type B influenza viruses.
Type A
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.
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.
Battling bird flu
Birds can get the flu, too. They can get really sick or just a little under the weather, just like us. Sometimes they can pass the flu to us. What makes birds really sick may not make us really sick, but a flu that’s nothing for a bird may be catastrophic for us.
Many viruses don’t last when they try to infect another species, and even if they infect us, we may not be able to spread the virus to others. But there are specific subtypes that are usually found in birds that, when they cross to humans, can be quite deadly. These subtypes are commonly called bird flu or avian influenza. Bird flu, though rare, can be fatal. It’s fatal for some common types in about half of cases (H5N1 [60 percent], H7N9 [40 percent]). Fortunately, these bird flu subtypes don’t have much person-to-person spread.
Suffering from swine flu
H1N1, another type A flu, has been a big worry for us in the past. An H1N1 subtype was the cause of the 1918 influenza pandemic (also called the Spanish flu), which led to the loss of more than 50 million lives worldwide and 650,000 in the United States. This subtype had genes of avian origin, though we haven’t yet figured out where it originated.
H1N1 has made news again and more recently caused concern when it began to spread in 2009. The virus crossed over from pigs, which is how it got its name: swine flu. However, H1N1 isn’t the only subtype of influenza to originate in pigs; the 2009 version, ultimately dubbed A(H1N1)pdm09, was, however, a new and unique variant not specifically seen before in animals or people, although it was related to prior H1N1 outbreaks.
Swine flu influenza viruses are spread, like other influenza viruses, through droplets in the air or by touching something that, for a graphic example, your pig has sneezed on, and then touching your mouth, nose, or eyes. While the first few cases were found in people who had direct or close contact with pigs, later cases were found to be from person-to-person contact. A vaccine was developed by the end of the year, and the pandemic ended by August 2010, according to the World Health Organization (WHO). In the United States, the CDC estimated there were about 60 million cases and 12,469 deaths from swine flu within the year after it was discovered, with worldwide deaths estimated at around 150,000 to 575,000.
Type B
Type B causes illness similar to Type A. Type B doesn’t have subtypes but has two main lineages. It isn’t as common a cause of illness as type A is. Generally, type B causes just 25 percent of influenza illnesses during the year. It doesn’t change by shift, only by drift, so it doesn’t have the same sudden changes that throw off our immune system. Type B also isn’t known to infect animals, except for seals, so we don’t have to deal with lineages from animals. Type B can still cause serious illness, including pneumonia, and can be fatal in some cases. Vaccines used to include just one B influenza virus, but now most include two.
Type C
Type C usually causes milder illness and is more common in children. Health providers don’t vaccinate against Type C and usually don’t test for it. You may well have had influenza type C in the past and not recognized it, because symptoms are generally mild. People and some animals are susceptible to this strain of influenza.
