The question about Covid-19 reinfection and an immune response is important not only for public health response to the pandemic but also for the administration of the vaccine.

As countries roll out the vaccine against the novel Coronavirus, one question that has been asked repeatedly is about reinfection. In December 2020, the World Health Organization warned that though reinfection was rare, there was still a possibility that a person could become reinfected once the antibody response

Dr. Mike Ryan, executive director of the WHO’s health emergencies program, at a press conference in December 2020, pointed to data emerging that showed that “protection may not be lifelong, and therefore we may see reinfections begin to occur”.

Since the first cases of COVID-19 were reported in December 2019, the pandemic has led to nearly a billion cases globally and over 2.1 million deaths. A look at the available data and research proves that reported reinfections, especially symptomatic, are rare. The immune protection has been seen to last for at least six months. The question about how long the immune response lasts is an important question not only for the public health response to the pandemic but also for the administration of the vaccine.

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The first case of reinfection in Hong Kong

The first case of Covid-19 reinfection was identified in Hongkong, in a 33-year-old male. The University of Hong Kong’s Li Ka Shing Faculty of Medicine Department of Microbiology which investigated the case found that genome sequencing showed the reinfection was caused by a different virus strain than the one that caused the infection the first time around.

Reinfection cases were also reported in a 25-year-old male in Nevada, US, a 51-year-old female in Belgium and a 46-year-old male in Ecuador. While the Hong Kong man was asymptomatic the second time around and the Belgium case was a mild one, both in the case of the Nevada and Ecuador men, the second infection was worse than the first one. Subsequently, several countries have reported reinfections, including India, Israel, Mexico, Brazil, Peru, South Korea, Belgium, Sweden, Spain, Netherlands, and Qatar.

Dutch website BNO News is tracking COVID-19 reinfections across the world. It has reported around 46 confirmed and 9,209 suspected cases of reinfection as of January 27, 2021. The average interval between the first incident and the second one was around 80 days.

Data published by the public health department in various states in the United States have indicated suspected cases of COVID-19 reinfection. The Washington State Health Department which has registered around 298,000 cases said that 681 cases had met its criteria for investigation of possible reinfection. Colorado (381,000 total cases) and Indiana (601,000 total cases) have similarly reported 300 and 250 cases of suspected reinfection.

South Africa, which is dealing with a new mutant strain of the virus has registered around 1.1 million cases, reported 4,326 suspected cases of reinfection in January 2021.

What do we know about immunity?

Most recent research has shown that COVID 19 infection does grant immunity for a period ranging from at least six months according to WHO and up to eight months, as per research by researchers at Monash University in Australia.

Monash University studied around 25 COVID-19 patients – their blood samples were collected from Day 4 post-infection to Day 242. While antibodies against the virus started to wane after Day 20, the researchers noticed that all patients continued to show the presence of memory B cells that recognised both the spike and the nucleocapsid proteins of the virus for up to eight months.

A larger study conducted in the UK among health care workers has answered some more questions about immunity as well as reinfection. The pre-print was published by Sheila Lumley and researchers at the University of Oxford. The researchers studied around 12,219 healthcare workers for around 30 weeks between March and November. When the study started 11,052 had returned negative tests and 1,246 had positive antibody results. The positive antibody results meant that they were presumed to have been infected with the severe acute respiratory syndrome coronavirus 2 (SARS-COV-2) but were not in the infectious phase. According to the Centre For Disease Control and Prevention, US, antibodies to SARS-CoV-2 start developing one to three weeks after infection.

Of those who were negative, around 223 had tested positive subsequently. Of 1246, positive cases, follow up testing on an average between 160-199 days, three seropositive health workers (meaning who had tested positive for the presence of antibodies previously) had returned PCR-positive tests (meaning that SARS-CoV-2 infection had been detected).

Reinfections are rare

Immunologist and Associate Professor Menno van Zelm from the Department of Immunology and Pathology at Monash University led the research with Covid-19 patients in Australia. In an email interview with Health Analytics Asia, he explained that there is still a lot to learn about immunity and reinfection when it comes to the COVID-19.

“Reinfection means that an individual after fully clearing a pathogen becomes infected again upon re-exposure. This is quite uncommon because humans have an advanced immune system that includes an adaptive arm. Following the first infection, the cells and antibodies of the adaptive immune response that are specific to this pathogen will remain in the body and remember the original pathogen. Thus, upon re-exposure, these can prevent infection,” explained van Zelm.

Immune response might be the norm but not always and not with all viruses. “Sometimes, these cells cannot fully prevent reinfection, typically in upper airways, which are distant from the systemic immune response. However, they can act quickly and prevent severe systemic disease,” said van Zelm.

The data reported in the past year of the global pandemic reveals that the immune response may not be life-long. “In the context of the large numbers of infection worldwide, the cases of reinfection are really small, and of these, most have a mild course of the disease. In this context, the immune response to the virus that causes COVID-19, SARS-CoV2, seems to behave similarly to other respiratory viruses,” the immunologist pointed out.

With vaccines being rolled out globally, there are questions on their efficacy against strains and addressing reinfection.

“Information about reinfection is important because it teaches us that the vast majority of people generate immunity to the virus after recovering from the first infection. Hence, a vaccination strategy is valid as this is aimed to trick the immune system by generating the same immunity to non-infectious parts of the virus. Indeed, phase III trials of the Pfizer and AstraZeneca vaccines demonstrated that infections in the vaccinated group were rare and much lower than in the placebo control group,” said van Zelm.

“Still, it is extremely important to continue follow-up of previously-infected individuals as well as vaccinated individuals as we do not know yet how durable immunity is and whether this longevity is different between infection and vaccination, or even between vaccines,” added van Zelm.

A year into the pandemic, the jury is still out on the prevalence of reinfection. What we know for sure is that patients infected with the COVID-19 virus are granted immunity for some months and reinfection is rare. That is hopeful news as the world gets vaccinated to stop the pandemic. Questions remain about the long-term immune response as well as the efficacy of the vaccines against new mutant strains of the virus.