Clinicians have been concerned that COVID-19 vaccines would lessen the effectiveness of drugs used to treat nasopharyngeal cancer, a type of cancer that affects the throat, but a new study shows the opposite: The COVID-19 vaccine actually helps with treatment.

Nasopharyngeal cancer is often treated with immunotherapy known as anti-PD-1 therapy, which blocks PD-1 receptors on the surface of immune cells whose job it is to hunt down invaders.

Blocking these receptors frees immune cells to do their bidding, but cancer cells co-opt these receptors to effectively shut down the body's natural defense systems against tumors.

Since vaccination against COVID-19 stimulates the body's immune response via these same cell signaling pathways, scientists needed to know how COVID-19 vaccines might interact with this type of cancer treatment due to the similarities between the drugs.

"It was feared that the vaccine would not be compatible with anti-PD-1 therapy," says bioinformatics scientist Jian Li from the University of Bonn in Germany.

"This risk is especially true for nasopharyngeal cancer, which, like the SARS-CoV-2 virus, affects the upper respiratory tract."

The team analyzed the records of 1,537 patients being treated for nasopharyngeal cancer across 23 hospitals. Of that cohort, 373 individuals had been vaccinated with the SinoVac COVID-19 vaccine used in China before starting their cancer treatment.

"Surprisingly, they responded significantly better to anti-PD-1 therapy than the unvaccinated patients," says Christian Kurts, an immunologist at the University of Bohn.

"Furthermore, they did not experience severe side effects more often."

For the time being, we don't know why this is happening; more research is required to assess the biological and chemical processes causing patients vaccinated with SinoVac to respond better to anti-PD-1 therapy.

"We assume that vaccination activates certain immune cells, which then attack the tumor," says Qi Mei, a cancer researcher at Shanxi University Hospital in China. "We will now investigate this hypothesis further."

One benefit of the study is that patients from so many different Chinese hospitals were involved in it, so a variety of demographics and regions were covered. However, the data is only for one type of vaccine and one type of cancer.

While nasopharyngeal cancer is rare in countries such as the US and the UK, it's widespread in southeast Asian countries and southern China. Scientists think the frequent use of air conditioning and nutritional factors might play a role, and it has also been linked to the Epstein-Barr virus.

In Taiwan, the disease is now one of the leading causes of death among young men, and the hope is that this link between improved treatment effectiveness and COVID-19 vaccination might help develop better ways of tackling the cancer.

"Future studies are warranted to elucidate underlying mechanisms," the researchers write in their published paper.

"The association of COVID-19 vaccination with increased efficacy of anti-PD-1 therapy with chemotherapy in recurrent metastatic nasopharyngeal cancer is interesting, but needs to be validated in a larger cohort study."

The research has been published in the Annals of Oncology.