Antibodies produced in the nose decline nine months after COVID-19 infection, while antibodies found in the blood last at least a year, a new study has revealed.

Antibodies in the nasal fluid (known as immunoglobulin A or IgA) provide first-line defence against COVID-19 by blocking the SARS-CoV-2 virus when it first enters the respiratory tract.

These antibodies are very effective at preventing the virus from entering cells and causing infection. However, the investigators from Imperial College London found that the nasal antibodies were only present in those recently infected and were particularly short-lived against the Omicron variant compared to earlier variants.

These new findings -published in eBioMedicine - may explain why people who have recovered from COVID are at risk of reinfection, especially with Omicron and its sub-variants.

The study also found that vaccination is very effective in creating and boosting antibodies in the blood, which prevent severe disease but had very little effect on nasal IgA levels.

"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 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," he added.

The research was led by teams from Imperial College London and the University of Liverpool. It studied almost 450 people who had been hospitalised with COVID-19 between February 2020 and March 2021, before the emergence of the Omicron variant and prior to the vaccine rollout.

The study also found that while current vaccines are effective at boosting blood antibodies, which can prevent serious illness and death, they do not significantly boost nasal IgA antibodies.

The researchers call for the next generation of vaccines to include nasal spray or inhaled vaccines that target these antibodies more effectively.

"Our results highlight a need for nasal spray vaccines that can boost these local antibodies in the nose and lungs," said Professor Peter Openshaw from the National Heart and Lung Institute at Imperial College London.