Fortune spoke with Peter Hotez, dean of the National School of Tropical Medicine at Baylor College of Medicine and co-director of the Texas Children’s Hospital Center for Vaccine Development, about some of the most common rumors still circulating and what you need to know.

Myth 1: The mRNA vaccines change your DNA

The Pfizer/BioNTech and Moderna vaccines use mRNA technology to deliver a snippet of viral code to your body in order to teach your immune system what the SARS-CoV-2 virus looks like. If your immune system then encounters the virus later, it’s ready to mount an immediate defense thanks to specialized antibodies and T cells designed to attack the virus.

The mRNA in the vaccines gets taken into an area of your cells called the cytoplasm. There, the mRNA is turned into proteins—specifically, the infamous coronavirus spike protein the virus uses to infect cells. It is this protein that primes the immune system to spring into action if you later come into contact with the real thing. The mRNA does not cross the barrier inside the cell where the DNA is stored, which is called the nucleus.

“This is why we took high school biology. We learned that the cell has a nucleus and cytoplasm, and the DNA is in the nucleus,” says Hotez. “[The mRNA] does not go into nucleus,” so it can’t change your DNA.

Myth 2: The vaccines negatively affect fertility

Anti-vaccine groups first started this rumor not about the Covid-19 vaccines but about the HPV vaccine, which protects against cervical cancer. Several large studies have debunked this initial claim, showing there is no data to back it up. There is no reason to believe that it is true for the Covid-19 vaccines either.

“They just copy-pasted it onto Covid-19 vaccines,” Hotez says. “It was not true for the HPV vaccine, and it’s not true for Covid-19 vaccines.”

Unfortunately, this myth has led to many pregnant people not getting vaccinated, despite the fact that they are at a significantly higher risk for severe disease and death from Covid-19. Multiple studies have shown that the vaccine is just as safe and effective in pregnant people as it is in non-pregnant people.

There have been anecdotal reports that the Covid-19 vaccines can affect women’s periods right after they get vaccinated, and the National Institute of Child Health and Human Development is now funding research into this potential side effect. Gynecologist Jen Gunter wrote on her blog that the vaccines could theoretically have this effect by impacting the lining of the uterus, which is part of the immune system and gets shed every month during a woman’s period. However, right now that is just a theory. Other possibilities are that the changes are just a coincidence and aren’t connected to the vaccine, or that stress from the pandemic and getting vaccinated are affecting women’s menstrual cycles. Notably, Covid-19 itself has been shown to alter menstrual cycles in roughly 25% of women.

Myth 3: The vaccines were rushed, and we don’t know what the long-term side effects will be

While the SARS-CoV-2 virus has been in existence for less than two years, research on mRNA and coronavirus vaccines has been going on for decades. The first scientist to propose using mRNA as a drug did so in 1988. The first test of an mRNA vaccine in mice occurred in 1993, and the first clinical trial of mRNA vaccines in humans started in 2015.

“We invested a decade of research and development in our coronavirus vaccine program showing that the spike protein was the target of the virus, how you deliver the spike protein, how you measure virus neutralizing antibodies. So there was at least a decade of research there,” Hotez says. “And then the discovery that mRNA could become a molecule that can induce the immune response goes back to the early 1960s. So in some ways, it’s the convergence of a decade or more of research.”

As for the Covid-19 vaccines themselves, the first clinical trials started in March 2020, now over 18 months ago. Paul Offit, director of the Vaccine Education Center at Children’s Hospital of Philadelphia, has stated in multiple interviews that, historically, the most severe side effects of vaccines have all occurred in the first six weeks after dosing. In other words, the risk of a serious side effect emerging after a year and a half is slim to none. 

Myth 4: If you already had Covid-19, you don’t need a vaccine

“The refrain is natural immunity is better than vaccine immunity, and it’s false,” Hotez says.

While it’s true that if you already got infected with the SARS-CoV-2 virus you’ll most likely make antibodies against it, you don’t know how many or how long they’ll last. Studies have shown that antibody levels among people with past infections can vary dramatically, and Hotez says that “you don’t know what group you fall under.” The vaccines, on the other hand, consistently show that they result in very high antibody levels (the exception being if you’re immune compromised). 

What’s more, antibodies—from both the vaccine and natural infection—wane over time, so if you have low antibody levels to begin with, you could lose much of your protection in just a few months. 

However, if you’re infected and recover and then you get vaccinated, you develop really robust protection, superior to either infection or vaccination alone. In other words, if you’re infected and recovered, you should definitely still get vaccinated.

Myth 5: The vaccines don’t protect against transmission

When the vaccines were first released, the focus was on their protection against symptomatic illness, hospitalization, and death. Later, studies from Israel showed that they also helped prevent transmission by reducing virus shedding if someone experienced a breakthrough infection. 

It’s true that vaccine efficacy against infection wanes over time, and the Delta variant that took hold this summer causes people to produce higher quantities of viral particles. As a result, protection against transmission in vaccinated people is not as strong as once it was. But that doesn’t mean it’s completely disappeared.

For one thing, you’re still at least 50% (and probably more than that) less likely to get infected with SARS-CoV-2 if you’re vaccinated than if you’re not, which means your risk of transmission is also cut in half. Simply put, if you’re not infected, you can’t transmit. Also, vaccinated people appear to shed virus for a much shorter period of time than unvaccinated people, so the number of days they’re infectious is fewer. Finally, full protection against infection and transmission appears to be restored if you get a booster shot. 

“It should have been explained from the beginning, this is a three-dose vaccine,” Hotez says. “And what’s going to happen is as vaccine immunity wanes, you might lose that property for a while until you get boosted.”