Surviving Covid-19 or receiving preliminary vaccines gives monkeys protective immunity to SARS-CoV-2. Although we don't yet know how long the protection lasts, or if the same will be true in humans, the work bolsters hopes for a vaccine and lowers the chances our worst fears about the virus will be realized.
Our immune systems retain memories of the enemies they have overcome. For most, but not all, viruses, the remnant antibodies allow us to fight the disease off quickly if exposed again. So one of the most critical questions regarding any new infectious disease is whether immunity is generated in survivors, and if so, how long it lasts. Among other things, without immunity to the natural disease making an effective vaccine is much harder, although not necessarily impossible.
Since people who have survived Covid-19 are seldom keen to test their immunity by exposing themselves to the virus again, research has turned to animals. A team led by Professor Dan Barouch of Harvard University infected nine adult rhesus macaques with SARS-CoV-2 and tracked their symptoms and viral load thereafter.
Although three different doses were used, all the monkeys' viral loads peaked on day two at around the same levels and declined thereafter to become undetectable between days 21-28. The monkeys did not have a happy time of it, developing pneumonia and inflammation in various organs, but none suffered respiratory failure and all recovered more quickly than most humans with serious cases of Covid-19.
Barouch's team reports in Science that 35 days after the original infection, they gave the monkeys the same doses of SARS-CoV-2 as each had received initially. None responded the same way. Instead their viral loads started low and declined rapidly.
Of course it would be vastly better if immunity could be generated by a vaccine. Many of the same scientists published another paper in Science on monkey responses to vaccines. The team developed six DNA vaccines that expressed various forms of SARS-CoV-2's famous “spike”, which it uses to invade cells and spread them between 25 macaques, re-vaccinating three weeks later.
Six weeks after the original vaccination these monkeys, along with 10 unvaccinated controls, were exposed to the virus. Eight of the vaccinated animals produced no detectable viral RNA of their own, and the rest much less than the controls.
With only four or five animals getting each vaccine, comparisons need to be treated with caution, but the monkeys receiving certain vaccines showed dramatically lower levels of virus throughout their bodies compared to the controls. Other vaccines were associated with lower virus detection in the lungs, but not in nasal swabs. “These data suggest that it may be easier to protect against lower respiratory tract disease compared with upper respiratory tract disease,” the researchers write.
Many potential vaccines that appear promising in animals fail in humans, which is why we go through the expense and delay of clinical trials, but success in a fellow primate is a better indication than in more distantly related animals. The specific vaccines used in the second paper are preliminary, and may require considerable modification, but Barouch said in a statement, "Our findings increase optimism that the development of COVID-19 vaccines will be possible.”