Lyme disease is on the rise. But scientists are searching for ways to prevent it.
Each time I pull out a wriggling tick from my dog's fur, I flash back to my own brush with Lyme disease – a bacterial infection spread via tick bites. I never found the deer tick that bit me, but I remember the flu-like chills, the body aches and signature bullseye rash that ran up and down my arm. A month of intense antibiotics cleared the infection and I haven’t had any further problems, but other people aren't so lucky.
In addition to an oval or circular rash, Lyme disease can cause flu-like symptoms. Infections that go untreated in the short term can lead to potentially debilitating issues in the months and years that follow, such as fatigue, body aches or nerve damage.
Meanwhile, there are an estimated 476,000 cases of Lyme disease diagnosed in the US every year – and as ticks expand their territory, that risk is growing. In the US, there are currently no licensed vaccines to prevent Lyme disease in people – but that might be about to change. And scientists are developing other strategies to help prevent infections, too. This is the battle against a group of particularly resilient bacteria and their spider-like host – and it's heating up.  
"We are seeing many more patients, many more people actually getting infected," says Thomas Hart, assistant professor of molecular microbiology and immunology at Johns Hopkins Lyme and Tickborne Diseases Research and Education Institute, Maryland. "It's a good time for there to be a new preventative."
Tick populations are expanding their range throughout Europe and the US, thanks to expanding forest and deer populations and climate change creating milder winters. And while tick season usually stretches from spring to autumn in many places, climate change is now increasingly leading to longer, sometimes year-round tick activity.   
Ticks carry a number of pathogens that can infect people, leading to diseases that include encephalitis, anaplasmosis and babesiosis, among others. But Lyme disease, caused by some types of Borrelia bacteria, is the most common tick-related illness in Europe and North America.
To this day my labrador Walter continues bounding around in long grass, unscathed by the illnesses ticks carry. He is vaccinated against Lyme disease and eats his monthly chewable tablet, smothered in spray cheese, to ward off fleas, ticks and heartworm – a disease spread via mosquito bites.
Currently there are several anti-tick treatments for dogs and cats, which often rely on chemicals that kill ticks once they start to feed. Typically these are either applied directly to the skin as a "spot-on" treatment, disseminated via a collar, or delivered as a tablet.  
Developing preventative drugs for humans, on the other hand, is no easy task. Studies in people are harder and more costly to run than those in animals. For trials in people, scientists often need to recruit a large number of participants and follow them over time as they go about their lives. That's because only a small fraction of them will get bitten and develop a tick-borne disease, and there need to be enough infections to show if the vaccine or drug works.
In animal trials, meanwhile, researchers often put infected ticks directly on them, almost ensuring that they will get infected with a given tick-borne pathogen, making the trials easier to do. But scientists hesitate to use so-called challenge trials for Lyme disease in people, in part because they don't want to risk giving people lingering, long-term symptoms from the disease.
"I don't think anyone would sign up for Lyme," says Linden Hu, professor of immunology at Tufts University School of Medicine in Massachusetts, who has worked with several companies on anti-tick strategies for humans. He explains this is why it's hard to get them approved.
There was, however, a human Lyme disease vaccine approved in the US in 1998, called LYMErix. The shot was tested in a trial with more than 10,000 participants and had 76% efficacy in preventing the disease. But demand was low, partly because of reports and lawsuits over side effects, namely arthritis. Health officials reviewed the data in 2002 and didn't find evidence of a safety risk. Still, the vaccine's maker discontinued the shot the same year, citing a lack of demand.
Now, the growing number of people at risk of tick-borne diseases has spurred a newfound investment in prevention. In particular, the search for another vaccine against Lyme disease is gathering pace – and starting to yield results.
So far, the vaccine candidate that's the furthest along is PF-07307405, by Pfizer and Valneva. It shares a similar target with the old LYMErix vaccine and the shots designed for dogs – a piece of a protein called OspA on the surface of the Lyme disease-causing Borrelia burgdorferi bacterium.
The shot prompts a person's immune system to create antibodies against six common versions of OspA found in the US and Europe. Then, when an infected tick bites the person and feeds on blood, it also takes in the person's antibodies. Inside the tick, the antibodies latch onto OspA, blocking the bacteria from leaving the critter.
"Your antibodies are able to fight off the bacteria when it's still inside of the tick, preventing infection in the first place," Hart says.
Earlier this year, Pfizer announced the results from a phase 3 trial – where scientists test how a new intervention compares with existing ones or a placebo and check its safety. In this case, the trial involved more than 9,400 people, revealing that a four-dose regimen was more than 70% effective in preventing Lyme disease. Fewer participants developed Lyme disease during the trial than expected, however, which meant results wasn’t quite as conclusive as the team had hoped.
No major safety concerns were detected, and Pfizer said in a press release that it is confident in the vaccine's potential and plans on submitting it to US government regulators for potential approval. 
"I will definitely get it as soon as it's available," says Laura Goodman, an assistant professor and emerging infectious diseases researcher at Cornell University College of Veterinary Medicine, New York.
Similarly, Moderna is testing two Lyme disease vaccines that target OspA. Both are in phase 2 trials – where an intervention is tested on a larger group of people than in phase 1, to see if it's safe and how well it might work. Meanwhile, researchers at Yale University are investigating a shot that prompts the immune system to respond to tick saliva. This one has not yet been trialled in humans.  
Another option is to skip the vaccine altogether. For more than a decade, Mark Klempner, a professor of medicine at UMass Chan Medical School, Massachusetts, has been developing an injection designed to target OspA. While a vaccine works by triggering the immune system to create antibodies, this delivers pre-made antibodies directly into a person. It takes just two days to absorb enough to be protected for four months.
Lyme disease is not found everywhere that there are ticks. It's rare to catch it in the southeastern US, for example, where ticks prefer to bite lizards over mammals. The idea behind the injection is that people who live, work or travel in areas with Lyme disease could get it at the beginning of tick season, Klempner says, plus a second dose later on.
The product is known as TNX-4800, and has already made it through phase 1 clinical trials, where an intervention is tested to find a safe dose and identify potential side effects. Tonix Pharmaceuticals licensed it in 2025 and plans on launching a phase 2 clinical trial next year, according to the company website.
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Another pharmaceutical company, Tarsus Pharmaceuticals, meanwhile, is working on a pill that deploys the same ingredient as an anti-tick medication for dogs. The drug TP-05 interferes with the ticks' nervous systems and kills them quickly, hopefully before they have time to transmit Lyme disease-causing bacteria and other pathogens.
In canines, the drug comes in the form of a chewable tablet, but in people it will be a pill. In March this year, Tarsus announced that it had started a phase 2 clinical trial in people, with results expected in the first half of next year.  
"I think it's going to be a game-changer," says Durland Fish, a professor emeritus of epidemiology at Yale School of Public Health, Connecticut, who has researched vector-borne diseases including Lyme disease for decades. The pill could also prevent other rising tick-borne diseases including babesiosis and anaplasmosis, he says.
But it's unclear when and if these options will become available. In the meantime, the best prevention is to stop tick bites in the first place. That means staying out of tall grass, wearing protective clothing, and showering and checking for ticks after coming inside. Those tried-and-true methods really work, Hu says, but they can be hard for people to do.
"People get tired or are out late at night, the kids are cranky," Hu says. That's where a vaccine or pill that people can take – but don't have to remember every time they go outside – could "really be a public health benefit," he says.
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