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Vaccines teach your immune system to recognize pathogens, but sometimes your body needs a bit of a reminder.

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Head to   or click the link in the description to  get 50% off your first month of any crate. [♪ INTRO]. So you show up for your annual physical and the  doctor tells you you need a ten-year booster shot.

But you’ve already had all  your routine vaccinations,   so what’s in that syringe that’s so important? Well, the vaccines we have all do a  pretty good job of introducing our   bodies to certain diseases and convincing our  immune systems to take that threat seriously. But for some diseases, it takes  extra convincing.

Which is why   you might need to get the same shot more  than once -- weeks or even years later. Vaccines teach our immune systems to recognize  certain pathogens, or disease-causing agents. In a nutshell, they trigger the formation  of specialized cells and antibodies that   recognize the distinctive proteins that  stick out of pathogens -- called antigens.

If something with one of those antigens shows  its face, the antibodies latch on and disable it,   while those specialized cells  help to kick it to the curb. Now, there are a few vaccines that  require multiple doses of the same   shot. And there are almost as many reasons  for that as there are different vaccines.

Because how much immunity you get from a single  shot, and how long it lasts, can really vary. Some vaccines, like the Hepatitis  A vaccine, tend to require   multiple doses to dial immunity up to the best  possible levels when you first get them. Others, like the shot that  protects against tetanus,   require getting the same shot years  later to dial immunity back up to full   throttle -- what we often call a booster  shot, though the terminology can vary.

And scientists are learning that  some vaccines, like the HPV vaccines,   may provide solid immunity for  more than a decade with just   one shot -- even though their efficacy  was first established using multiple doses. Like I said, immunity can really vary. Immunology is notoriously complicated, and  we’re still trying to understand the details   of how our existing vaccines  work on a molecular level.

So, we’re still working out the right  number of shots and when to give them.   In some cases, that even applies to diseases  we’ve been vaccinating against for decades. But in general, you might need to go  back for a top-up because immunity   doesn’t always last forever. Basically, over time, immunity to some  threats can get sluggish as immune memories   get fuzzy and the numbers of  specialized cells dwindle.

It seems to happen in tiny  sites called germinal centers.   These form in the lymph nodes and  spleen upon encountering a new antigen. These centers are where immune cells  hang out, maintain their numbers,   keep their memories sharp, and lie in wait  in case their pathogen-of-interest shows up. Those cells include the B cells that  make antibodies, as well as T cells,   which are also thought to  help remember past invaders.

When germinal centers form, specialized  cells grab copies of the antigen   and B cells get to work testing out antibodies,  keeping only the ones with the tightest grip. How well immunity builds up in the first place,  and the extent to which it wanes over time,   might depend largely on how  efficiently this process goes down. Immunologists are also still figuring out how  time and the elements take a toll on how well   older people’s immune systems work to fight off  pathogens that they’ve been vaccinated against.

Researchers are still working on how to  tweak different vaccines to induce the   formation of the best germinal centers to  get immunity to last as long as possible. That said, we’re still  learning how this all works! Overall, though, whether we’ll need booster  shots down the line depends on a few things,   like how much our immunity declines.   For example, people who contract measles  once are considered immune for life.   And so are those who receive a pair of shots  of the vaccine we’ve been giving since 1967.

Many people -- though not all -- will develop  a robust immune response after just one shot. The second dose just ensures that more  people will be thoroughly protected. Measles surface proteins aren’t good at mutating,   so the antibodies you make will  keep recognizing them indefinitely.

But with other vaccines, our immune  system needs a lot more help. Like pertussis, better known as whooping cough. In the 1980s, many countries started  switching to a new version of this vaccine   that only contained pieces of the  pathogen, instead of the full thing.

That was because the older  vaccine that contained killed,   whole cells of the pathogen had  occasional nasty side effects. The problem is, researchers  have realized more recently   that immunity from this cell-free  vaccine wanes after just two years. The current vaccination schedule calls  for kids to get their first shot a few   months after their first birthday, and  the second one after they turn four.

So it looks like our vaccination  schedules will need to be updated   to keep people protected from pertussis outbreaks. Now, what about the vaccines on  everyone’s mind -- the ones for COVID-19? Many of these vaccines are administered in  a two-shot series, so the question is what,   if anything, we’ll need after that.

And right now, we’re just not sure.  But scientists are working on it. One manufacturer, Moderna, plans to begin testing  booster doses of their current mRNA vaccine in  . July 2021, a year after the first clinical  trial participants received their shots.

Because it might be that we need boosters to rev  up waning immunity over time, like with tetanus. To cover the bases, Moderna is also working  on a novel booster candidate against   new strains that have emerged since  the initial vaccine was developed,   just to be sure our immune  systems also recognize those. But it’s also possible that the virus might  continue to mutate in ways that make current   vaccines less effective, requiring  a system more like the flu shot.   Yearly flu shots aren’t so much  boosters as whole new vaccines.

In short, we’re going to need a lot more  information before we know exactly how often we’re   going to need to be vaccinated against COVID-19 --  and how much those vaccines will need to change. The good news is, scientists are on the  case, so we won’t be left hanging forever. Like I said earlier, immunology is complicated.

But we’re getting better   against COVID-19 -- and all the other pathogens that cross our path. Thanks to KiwiCo for supporting  this episode of SciShow. KiwiCo creates incredible hands-on  projects for kids.

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