One of the largest studies conducted on COVID immunity has found that previous infections provide protection for as long as vaccination, but experts have cautioned that vaccines are still the safer option.
The study published in the Lancet journal found that even 10 months after getting COVID, people still had an 88 per cent lower risk of a reinfection resulting in hospitalisation and death.
That makes this natural immunity "at least as durable, if not more so" than two doses of Pfizer or Moderna's vaccines for ancestral, Alpha, Delta and Omicron BA.1 variants, the study said.
Funded by the Bill and Melinda Gates Foundation, the study did not include data on infection from Omicron XBB and its sublineage.
The authors emphasised that their findings should not discourage vaccination, which remains the safest way to get immunity.
Led by the US-based Institute for Health Metrics and Evaluation (IHME), the study said it was the most comprehensive analysis of how long protection lasts for the different forms of immunity.
Researchers reviewed 65 studies from 19 countries up to September 2022, meaning some covered the period when Omicron swept across the world.
Omicron proved to be more contagious than previous strains, but less severe.
People with natural immunity from a pre-Omicron variant saw their protection against reinfection wear off much more quickly for the early Omicron strains, dropping to 36 per cent after 10 months, the study said.
'Challenges with natural immunity'
Director of Infectious Diseases at Mater Health Services Paul Griffin, who was not involved in the study, said a greater understanding of immunity against COVID was still needed, so the extensive meta-analysis was a valuable addition to existing knowledge.
He said there was an obvious difference to keep in mind, however, between protection from vaccination versus that provided by infection.
"… infection carries with it the risk of disease (including severe disease in a proportion) whereas vaccination does not," Dr Griffin said.
"It is also important to consider that some of the challenges we face with the longevity of protection from vaccination — including for example waning over time and immune evasion — also apply to protection generated by infection.
Some of the challenges to also consider with natural immunity were evident in this study by the protection provided by natural infection being substantially lower for the Omicron variant BA.1 than pre-Omicron variants and that it also declined more rapidly than protection against previous variants."
'Hybrid immunity' should lower healthcare burden
Study co-author Caroline Stein of the IHME said vaccines continued to be important for everyone in order to protect high-risk populations such as those who are over 60 and those with comorbidities.
The study also gave a more accurate picture of what COVID might look like in the future, as more vaccinated people are reinfected, acquiring "hybrid immunity".
"In the long run, most infections will occur in people with strong protection against severe disease because of previous infection, vaccination, or both," epidemiologist at South Africa's National Institute for Communicable Diseases Cheryl Cohen said.
She said these results suggested that, "similar to other human coronaviruses, there might be a low seasonal hospitalisation burden" associated with COVID.
Findings can help public health policy
Molecular Virologist Dr Vinod Balasubramaniam also reacted to the study, saying the results could be beneficial in providing information that can be used to tailor guidance on the number and timing of vaccinations.
"At the public health level, these findings can be combined with data on local infection prevalence, vaccination rates, and their timing," Dr Balasubramaniam said.
"Interestingly, the level of protection from past infection by variant and over time is equivalent to that provided by two-dose mRNA vaccines has important implications for guidance regarding the timing of vaccine doses, including boosters.
"This finding also has important implications for the design of policies that restrict access to travel or venues or require vaccination for workers, where those with a documented infection should be treated similarly to those who have been fully vaccinated with high-quality vaccines."