(NEXSTAR) – As the delta variant makes COVID-19 more infectious than ever, do people who’ve already had it need to worry about reinfection?

“Yes and no,” said Dr. Peter Chin-Hong, an infectious disease specialist at the University of California, San Francisco. “Because immunity varies significantly from person to person after natural infection, there is no guarantee that a particular individual will have an immunological response.”

Basically, some people who have fought off COVID-19 develop a pretty strong immunity to it. Others only develop a weak immune response and therefore could be more vulnerable to infection. There isn’t really an easy way to tell which bucket you fall into.

The best way to battle that uncertainty is by getting vaccinated, said Chin-Hong. “Vaccines remain the most reliable way to get durable protection.”

While breakthrough cases are being reported around the country, data show the vast majority of cases – especially serious cases – are among the unvaccinated. Analysis by the Kaiser Family Foundation found unvaccinated people make up 94% to 99% of COVID-19 cases, depending on the state. The unvaccinated make up an even larger share of current hospitalizations and deaths.

“The breakthrough cases are a sideshow,” said Dr. George Rutherford, an epidemiologist at UCSF. “That’s not what’s driving (the surge) here. It’s being driven by unvaccinated people.”

While some breakthrough cases are expected, people who get vaccinated are much less likely to contract the virus, according to the CDC. Experts say people with existing immunity – be it from a vaccine or a previous infection – are also less likely to experience severe symptoms even if they do get infected with the delta variant.