Around March, virologists introduced the herd immunity theory as a potential measure in reducing COVID-19’s exponential growth curve.
In any context, herd immunity describes a community’s biological resistance to the spread of a contagious disease. This outcome is most often facilitated via the development of a vaccine.
For herd immunity to offer any meaningful relief during the coronavirus pandemic, resistance must emerge in a high proportion of populations for long enough to see recovery rates drastically outpace infection rates.
There isn’t really a consensus about how long infected patients remain protected from SARS-CoV-2 reinfection.
In the last three months, there seem to be two competing theories taking shape. The first suggests that protective immunity may be lost by six months post-infection. A promising analysis conducted on rhesus monkeys favored this appreciation of the available data sets.
The second theory favors the concept of transient immunity. I.e our immune response is geared against SARS-CoV-2 reinfection during and immediately after viral clearance but not for much longer than that.
Subscribers from both sides concede that the prospect of reaching functional herd immunity by natural infection seems very unlikely.
Stephen Griffin, Associate Professor in the School of Medicine, University of Leeds, explained of the reports: “Vaccines in development will either need to generate stronger and longer-lasting protection compared to natural infection, or they may need to be given regularly.
“Whilst studies in primates show relatively weak responses that protect against severe disease but not infection itself or subsequent virus production, recent studies in pigs show that additional inoculations might improve outcomes.”
A new yet to be reviewed paper submitted to the pre-print server medRxiv claims to have a better idea about the duration of our body’s targeted immune response to the novel coronavirus.
Antibodies for SARS-CoV-2 can be detected in the majority of infected individuals roughly ten to 15 days following the onset of COVID-19 symptoms. Some may theoretically benefit from these antibody responses longer than others, though all of the mechanisms have not yet been identified.
After examining blood samples taken periodically from 65 people who tested positive for coronavirus up to 94 days after they first had symptoms, the authors discovered that more than 95% of the patients had coronavirus antibodies in their blood. The severity of COVID-19 conditions among the study pool ranged from asymptomatic to serious enough to require life support.
After eight days, participants belonging to both ends of the spectrum evidenced antibodies capable of effectively neutralizing the SARS-CoV-2 virus.
However, after 23 days the average concentration of coronavirus fighting antibodies (nAb) peaked among the study pool. This figure continued to decline two to 23-fold during the 18 to 65-day follow-up period. In samples collected after 65 days, the percentage of participants who still had potent targeted antibodies subsequently dropped to 16.7%.
“This study has important implications when considering widespread serological testing, Ab protection against re-infection with SARS-CoV-2 and the durability of vaccine protection,” the authors wrote in the new paper. “We demonstrate that the magnitude of the nAb response is dependent upon the disease severity, but this does not affect the kinetics of the nAb response. Declining nAb titres were observed during the follow-up period. Whilst some individuals with high peak ID maintained titres some with lower peak ID50 had titres approaching baseline within the follow-up period.”
The severity of the COVID-19 illness may be educative of the amount of antibodies presence post-infection and clearance. Patients who demonstrated high levels of nAbs at their peak maintained them after more than 60 days, but those with lower levels returned almost to their baseline level.
Unfortunately, this does not seem to offer anything substantive about one’s ability to remain immune to the coronavirus.
To strengthen their findings, the authors paired their results with a collection of blood samples from 31 healthcare workers at a group of hospitals in the U.K. These patients experienced a similar drop in nAbs overtime.
Even though the correlation between how sick someone is and how many nabs they develop is pretty clear, more research needs to be conducted to determine if our bodies develop more antibodies to fight critical forms of COVD-19 or if the excess antibodies are making us sicker themselves.
Dr. James Gill, Honorary Clinical Lecturer at Warwick Medical School, U.K., and a practicing family doctor, said the study is a potential “warning shot to those who have been found to have antibodies to COVID yet have only had mild if any symptoms.”
“Even those with a positive antibody test, especially those who cannot account for where they may have been exposed, should continue with to use caution, social distancing, and appropriate mask use,” he concluded.