Researchers think they’ve figured out why people can become reinfected with COVID-19, despite immunity gained from either vaccination or a previous infection.

It turns out that antibodies produced in the nose — the first line of defense against respiratory viruses like COVID — decline faster than antibodies found in the bloodstream, British scientists say.

Nasal antibodies tend to drop nine months after COVID-19 infection, while antibodies in the blood last at least a year, according to findings published online Dec. 19 in the journal eBioMedicine.

The study also found that vaccination is very effective in creating and boosting blood-borne antibodies that protect against severe disease, but had very little effect on nasal antibodies.

“Before our study, it was unclear how long these important nasal antibodies lasted. Our study found durable immune responses after infection and vaccination, but these key nasal antibodies were shorter-lived than those in the blood,” said lead researcher Dr. Felicity Liew, from the National Heart and Lung Institute at Imperial College London.

“While blood antibodies help to protect against disease, nasal antibodies can prevent infection altogether. This might be an important factor behind repeat infections with the SARS-CoV-2 virus and its new variants,” Liew added in a college news release.

The new study evaluated nearly 450 people hospitalized with COVID-19 between February 2020 and March 2021, before the emergence of the Omicron variant and the rollout of COVID-19 vaccines.

The researchers analyzed antibodies taken from patients when they were hospitalized, as well as at six months and one year later.

More than two-thirds of the patients also received their first COVID vaccination during the follow-up period.

Vaccination led to increases in nasal and blood antibodies, but the change in the nasal defenses were small and temporary, the investigators found.

The next generation of vaccines should include nasal spray or inhaled vaccines that would boost nose antibodies more effectively, potentially reducing infections more effectively, the researchers concluded.

“Our current vaccines are designed to reduce severe disease and death, and are dramatically effective in this aim. It’s now essential to also develop nasal spray vaccines that can provide better protection against infection,” said co-senior researcher Peter Openshaw, from the National Heart and Lung Institute at Imperial College London.

“It’s brilliant that current vaccines mean fewer people are becoming seriously ill, but it would be even better if we could prevent them from getting infected and transmitting the virus,” Openshaw added.

More information

The U.S. Centers for Disease Control and Prevention has more about COVID vaccines.

SOURCE: Imperial College London, news release, Dec. 19, 2022