What do we know about Covid-19 immunity and reinfection?
Although patients who recover from Covid-19 will hope to have developed antibodies conferring protection against the virus, there remain questions about immunity and how long it lasts.
There have been a handful of examples of what are believed to be reinfections around the world, but a study published on November 20 gives grounds for hope that those infected are unlikely to be infected, or suffer symptoms of Covid-19, for at least six months after their initial infection.
The researchers cannot yet say if the protection last longer, which has implications for Boris Johnson, who battled the infection in April and was admitted to hospital with a severe case of the virus.
The Prime Minister is self-isolating, it emerged on Sunday night (November 11), after contact with Lee Anderson, MP for Ashfield, who has since tested positive for the coronavirus.
Here is what we know so far.
How long do antibodies last after Covid-19 infection?
There is uncertainty over this, because the virus is new. But a study published in the journal Immunity in October showed evidence that virus-neutralising antibodies in people who had been infected with Covid-19 were still detectable after five to seven months.
Those patients who had more severe cases were found to have higher levels of some of these antibodies, the study found.
But those who had recovered from milder cases had memory B cells and memory T cells. These give our immune system a ‘memory’ of previous infections, meaning it can respond more quickly and stronger should it re-encounter the same pathogen.
The researchers were hopeful that this meant that a majority of those who recovered from an infection would develop some resistance - at least until the virus mutates sufficiently to evade the immune system.
“Of nearly 30 million cases to date since December 2019, there have been only about 10 documented and confirmed cases of re-infections. These data suggest that resistance to reinfection might be less a function of durability of the immune response and more one of breadth,” said the authors, led by Deepta Bhattacharya, an immunobiologist at the University of Arizona College of Medicine.
“Although the SARS-CoV-2 genome is diversifying slowly compared with other more mutable pathogens, high levels of pre-existing immunity in communities could lead to the selection of rare viral variants that evade neutralising antibodies.”
But the study was limited, because the researchers did not know whether the individuals tested had been exposed again to the virus, and therefore whether their immune system was capable of fending off infection - or at least preventing severe symptoms.
What do we know about reinfections?
A study published on Friday November 20, suggested that individuals who had Covid-19 are highly unlikely to contract it again for at least six months following their first infection.
The study - part of a major collaboration between the University of Oxford and Oxford University Hospitals (OUH) NHS Foundation Trust - covered the 30-week period from April-November 2020 and involved 12,180 health care workers employed at OUH.
The health care workers were tested for antibodies against Covid-19 as a way of detecting who had been infected before, and were tested regularly for the virus.
- 89 of 11,052 staff without antibodies developed a new infection with symptoms. None of the 1,246 staff with antibodies developed a symptomatic infection.
- Staff with antibodies were also less likely to test positive for Covid-19 without symptoms - 76 staff without antibodies tested positive compared to just three with antibodies.
- The three healthcare workers with antibodies who tested positive for the virus were all well and did not develop symptoms of Covid-19 again.
One of the authors, Professor David Eyre, of the University of Oxford’s Nuffield Department of Population Health, said: “This ongoing study involving a large cohort of healthcare workers has shown that being infected with Covid-19 does offer protection against reinfection for most people for at least six months – we found no new symptomatic infections in any of the participants who had tested positive for antibodies, while 89 of those who had tested negative did contract the virus.
“This is really good news, because we can be confident that, at least in the short term, most people who get Covid-19 won’t get it again.
“We know from a previous study that antibody levels fall over time, but this latest study shows that there is some immunity in those who have been infected. We will continue to follow this cohort of staff carefully to see how long protection lasts and whether previous infection affects the severity of infection if people do get infected again.”
At the enough, there is not sufficient data to judge longer-term protection beyond six months from the initial infection, but the monitoring of the staff will continue.
There have been a handful of reports of suspected reinfections recorded in the USA, Hong Kong, Belgium, the Netherlands and Ecuador.
The first case of apparent SARS-CoV-2 reinfection in the USA involved a 25-year-old man from Nevada, who tested positive for a second time 48 days after the initial test, having tested negative twice in between.
A study published in The Lancet in October revealed that genomic analysis of the two virus samples from the patient showed significant differences. This suggested it was unlikely to be the first infection that caused the reinfection.
“It is possible that we have reported a case of continuous infection entailing deactivation and reactivation. However, for such a hypothesis to be true, a mutational rate of SARS-CoV-2 would be required that has not yet been recorded,” the study noted.
The second infection also caused a much more severe case, meaning the man, who had no underlying conditions, needed oxygen support in hospital.
The study, by Dr Richard Tillett and colleagues, said: “Previous exposure to SARS-CoV-2 might not guarantee total immunity in all cases. All individuals, whether previously diagnosed with Covid-19 or not, should take identical precautions to avoid infection with SARS-CoV-2. The implications of reinfections could be relevant for vaccine development and application.”
The authors concluded: “Our findings have implications for the role of vaccination in response to Covid-19. If we have truly reported a case of reinfection, initial exposure to SARS-CoV-2 might not result in a level of immunity that is 100 per cent protective for all individuals.
“With respect to vaccination, this understanding is established, with influenza regularly showing the challenges of effective vaccine design.”
The study was limited, however, because the researchers were unable to assess the immune response to the first episode of infection, or assess fully the effectiveness of the patient’s immune responses in the second.
The latest study from Oxford University gives us more confidence that there will be significant protection for the vast majority of those who have been infected.
How will we find out more?
Study of the evolution and spread of Covid-19, and better understanding of how mutations in its genome might affect treatments and vaccines, will be aided by the work of a consortium led by Cambridge scientists.
The Covid-19 Genomics UK (COG-UK) Consortium, led by the Wellcome Sanger Institute and University of Cambridge, this month passed the milestone of having completed the whole genome sequencing of 100,000 virus samples.
COG-UK, which includes UK public authorities and other partners, is helping to understand links between outbreaks.
It has just been granted a further £12.2million by the government to help expand its genomic surveillance efforts, following on from £20million granted when it was established in March.
Meanwhile, vaccine studies will also help to indicate how effective antibody responses are.
What does this mean for a vaccine?
The influenza vaccine has to be updated annually because different prevailing strains emerge.
That is not yet the case with Covid-19, which has so far proved more conserved, although there is evidence of mutation.
However, genomic surveillance will be important to understand its evolution and any implications for vaccines.
Akiko Iwasaki, writing in The Lancet in October, said: “There is currently no evidence that a SARS-CoV-2 variant has emerged as a result of immune evasion. For now, one vaccine will be sufficient to confer protection against all circulating variants.”
But the COG-UK Consortium has warned ongoing genomic surveillance will be vital to ensure this continues to be the case, particularly once a vaccine is introduced at scale, as more resistant strains may begin to fare better than others.
The consortium has established a group and system to explore such mutations.