Covid-19 study in monkeys gives insight into possible immunity

As the number of Covid-19 cases continues to increase around the world, many medical professionals and experts are questioning if patients will acquire protective immunity to Covid-19 post-infection and, if so, for how long. Such information is critical for epidemiologists to accurately map and predict the future spread of the virus, especially as a second wave of the pandemic is expected in late 2020.

On May 20, a study was published in Science about post-infection immunity to the Covid-19 virus, SARS-CoV-2, in rhesus macaque monkeys. A team of scientists led by Dr Dan Barouch at Harvard Medical School set out to determine if SARS-CoV-2 infection induces natural immunity that could protect against subsequent infections. This understanding is critical for vaccine development and public health strategies to protect against future outbreaks.

Nine adult monkeys were inoculated with the SARS-CoV-2 virus by an intranasal route, and the researchers monitored infection by assessing SARS-CoV-2 ribonucleic acid (RNA) levels via reverse transcription polymerase chain reaction (RT-PCR) testing. High viral loads were detected two days post-infection, indicating that viral replication had occurred. The monkeys also exhibited decreased appetite and responsiveness, suggesting that SARS-CoV-2 was having physiological responses on the animals. To determine if an immune response was occurring, the researchers quantified antibodies to the SARS-CoV-2 Spike (S) protein and were able to detect antibodies in all nine animals 35 days after infection.

Given that limited pathology data is available from SARS-CoV-2 infected humans, the research team assessed the pathologic features of SARS-CoV-2 infection in the rhesus macaques. Two days after infection, multiple regions of inflammation in the upper airway mucosae, trachea and lungs were observed. Inflammation also occurred in the respiratory epithelial submucosa of the larger airways and migration of inflammatory cells into the bronchiole lumen was detected. Four days after infection, inflammation and pneumonia had diminished but the virus was still detected in the lungs.

To determine the extent of naturally developed immunity, the scientists re-infected all nine monkeys with the same doses of SARS-CoV-2 that were used for the first infection 35 days prior. Very limited SARS-CoV-2 RNA was observed one day following the second infection, while no SARS-CoV-2 RNA was detected at subsequent time points. Additionally, no evidence of clinical disease was observed in the animals. All monkeys developed antibody responses, following re-infection.

The study is critical to determine whether or not an initial infection with SARS-CoV-2 can lead to immunity from future infections. While many viruses result in infected individuals developing virus-specific antibody responses and immunity from re-infection, not all viruses elicit protective natural immunity. Currently, there is no data available about whether human patients who have recovered from Covid-19 are protected from subsequent exposure to SARS-CoV-2.

In this study, Dr Barouch’s team demonstrated that SARS-CoV-2 infection in rhesus macaque monkeys provides protective immunity from second infections. However, the team did not reveal how long this natural immunity to SARS-CoV-2 lasts. Hopefully, this will be addressed in future studies. Clinical trials will need to be conducted in order to determine if SARS-CoV-2 infection results in natural immunity to protect against re-infection in humans.

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