Vaccination （接种疫苗） is among the most effective ways to help us prevent diseases. For viruses that don't change too much — the measles virus （麻疹病毒）, for example — getting vaccinated is a once-and-for-all method to prevent you from becoming infected with the virus. If you had two measles vaccines when you were a child, you would be protected for life.
However, it's a different story when it comes to influenza （流行性感冒）, commonly known as "the flu", which generally peaks between December and February. Flu vaccines cannot protect us in the long term.
There is no long-term immunity （免疫）, according to Theodore Strange, associate medical director at Staten Island University Hospital in New York. "The virus essentially'changes its coat' — H1N1, H2N3, and so on," he told the website Healthline.
Apart from a person's immune protection from flu vaccination declining over time, flu viruses are also constantly changing. So the vaccines are likely to be updated from one season to the next to protect against the viruses that research suggests may be the most common during the upcoming flu season.
Thus, to develop effective flu vaccines, over 100 national influenza centers around the world conduct year-round surveillance （监测） for influenza. Researchers will test thousands of influenza virus samples from patients, according to the official website of the US Centers for Disease Control and Prevention (CD). The World Health Organization also suggests there are three or four influenza viruses that are most likely to spread among people during the upcoming flu season.
But even when a vaccine is developed, getting it can prove to be difficult, as it is in high demand and short supply. The timing of influenza vaccine production and distribution is unpredictable, thus the availability of the flu vaccine supply does not always meet peak demand.
So scientists prioritize access to the vaccination. The CDC recommends key populations, such as medical staff, teachers, students, children and those aged 60 and above, receive flu vaccines.
This year the situation seems more complicated, as the upcoming flu season coincideswith the COVID-19 pandemic. This could overwhelm hospitals, straining （过度使用） esources and healthcare workers, time.
