A big question on the minds of clinicians and researchers alike is how long does immunity against SARS-CoV-2, the virus that causes COVID 19, last? Is it life-long? Is it a few months? If you get it once, does it mean that you can never get it again?
We still don’t know.
Enter this important study published in Nature Medicine. Researchers from China studied 37 asymptomatic individuals in the Wanzhou District who were diagnosed with confirmed SARS-CoV-2 infections but had no relevant clinical symptoms in the preceding 14 days and during hospitalization.
They found that IgG levels and neutralizing antibodies — the antibodies that are much longer-lasting and are protective against further infection — started to decrease within 2–3 months after infection. 40% of patients with asymptomatic infection had no antibodies detected within 8 weeks of the initial infection. And, it seemed that those who had no symptoms had a weaker immune response to the virus.
On the one hand, this makes some sense. If you didn’t develop any symptoms, your immune response may not have been as robust as someone else who had a fever, chills, body aches, and other symptoms. Many of those symptoms are as a result of the immune response to the virus itself.
On the other hand, just because it seemed that those patients who had an asymptomatic infection with SARS-CoV-2 had decreased antibodies after 2–3 months, it does not mean that they will get the virus again and again and again. This is because of the nature of the adaptive immune response to infection.
When we first get an infection — any infection — we develop an initial immune response with antibodies called IgM. Then, later, the body develops IgG antibodies, which more protective and longer-lasting. When we take vaccines, we are coaxing the immune system to develop those IgG antibodies.
Some immune responses, like that to Measles or the Measles vaccine, are life-long. Others, like that to influenza, do not last our entire lives. We still don’t know exactly what kind of immune response is generated to SARS-CoV-2. Still, even if the initial immune response doesn’t last very long, the body usually keeps a memory of the previous infection. And so, if and when we get infected with SARS-CoV-2 again, the body’s immune system should remember the previous infection, and it should develop a very robust secondary immune response.
The key word is “should.” If the virus mutates, then all bets are off, as the body has not seen this new mutated version of the virus. Further, what this study does not answer — and it cannot answer unless researchers deliberately infect these individuals with SARS-CoV-2 — is if these individuals with asymptomatic infection become symptomatic if re-exposed to the same virus again.
Still, the implications of this research are significant.
If immunity doesn’t last very long, then having antibodies doesn’t mean we are free to disregard social distancing requirements. And, if a vaccine is developed, perhaps it may need to be taken every year — like the influenza vaccine — and not once in our lifetimes, like the measles vaccine.
In addition, if it is true that 40% of individuals who had asymptomatic infection don’t have detectable antibodies 8 weeks after infection, if they test negative for the antibodies, it does not mean that they never had Covid-19.
This will make it harder to determine the true extent of viral infection in the community.
As I have written before, this virus and the infection it causes is so new, there is so much that we don’t know. Every bit of information adds to the larger body of evidence.
True, there is so much out there that it is overwhelming at times. That said, it is still important to gather that information. This is because, the more we know, the better equipped we will be to fight this horrible scourge.