New analysis suggests unvaccinated people could be reinfected with COVID-19 every 16 months on average.
A recent study at the Yale School of Public Health in Connecticut, US, published in The Lancet Microbe, analysed immunological data from close viral relatives of SARS-CoV-2, the virus that causes COVID-19, to determine how long natural immunity from infection lasts for unvaccinated individuals - indicating when they should expect to become reinfected with COVID-19.
The findings suggest reinfections are becoming more common as immunity against COVID-19 wanes. Professor Jeffrey Townsend and his colleagues noted that if there were no vaccines and no infection prevention measures, such as masking or social distancing, reinfection should be expected "on a three-month to five-year timescale - meaning that the average person should expect to get COVID every three months to five years".
In a model where everyone has either been infected with or vaccinated against the virus, unvaccinated individuals should expect to be reinfected with COVID-19 roughly every 16-17 months on average, the study found.
"Our results are based on average times of waning immunity across multiple infected individuals," study co-author Hayley Hassler told Yale Daily News. "Any one of those individuals may experience longer or shorter durations of immunity depending on immune status, cross-immunity, age, and multiple other factors."
As cases of the virus surge in England, people are reportedly catching SARS-CoV-2 - the virus that causes COVID-19 - for a second, or even a third time.
Scientists in the UK are now calling for the vaccination programme to be extended to schoolchildren, including two doses for teenagers, amid rising concerns over the impact reinfections could have on the National Health Service (NHS).
According to the NHS, people can get their first and second doses of a COVID-19 vaccine if they're aged 18 or over or will turn 18 within the next three months. Young people aged 16 and 17 and children aged 12 to 15 can currently only receive a first dose. A second dose may be moffered if a young person is at a higher risk of infection.
Stephen Griffin, an associate professor of virology at the University of Leeds, told The Guardian "high-level prevalence" and "frequent exposure" to the virus in schools will see more and more people getting reinfected - despite being double-jabbed.
Last year, research indicated although reinfections could occur, it was unlikely. However, new studies have since proven that immunity against the SARS-CoV-2 virus wanes over time.
Earlier this month, The Lancet Microbe published data showing the effectiveness of the Pfizer-BioNTech vaccine - the only COVID-19 vaccine currently available in New Zealand - at preventing infection dwindled from 88 percent to 47 percent six months after the second dose. The analysis also indicated the vaccine's effectiveness in preventing hospitalisation and death remained high at 90 percent for at least six months, even against the highly contagious Delta variant.
"If you don't clamp down on prevalence [in schoolchildren], you'll get the spread of infection and possibly reinfection, which will then potentially spread to parents whose vaccines may be waning, and more critically to grandparents and clinically vulnerable people," Griffin told The Guardian.
Danny Altmann, a professor of immunology at Imperial College London, told the newspaper that reinfection is now "far more common" than previously thought.
"Certainly in the healthcare workers that we've been studying, there are many people who had moderately decent levels of antibodies who have been, in some cases, previously infected and double-dose vaccinated, who have gone down with symptomatic infections."
Out of 20,262 Britons who tested positive for COVID-19 between July 2020 and September 2021, 296 were reinfected, according to data published on October 6 by the Office of National Statistics (ONS).
In the UK, reinfection is defined as a positive test 120 days or more after a first positive test, with an average of 203 days between the positive results. In the US, it is defined as a lab-confirmed case 90 days or more after a previous lab-confirmed infection.