Could I have had COVID and not realised it?
- Written by: Ashwin Swaminathan, Senior Lecturer, Australian National University
It seems not a day goes by without learning someone in our inner circle of family, friends and colleagues has COVID. When we ask how unwell our acquaintance is, the responses vary from “they’re really crook” to “you wouldn’t even know they had it”.
This is in line with studies that report moderate to severe illness in a minority of people (usually older with other risk factors) and that up to one in three positive people exhibit no symptoms[1].
Given the ubiquitous presence of this highly infectious coronavirus[2] in our community and the high rate of asymptomatic illness, those who have not been diagnosed with COVID might wonder, “how would I know if I had been infected?” And, “does it matter if I have?”.
Read more: Has Australia really had 60,000 undiagnosed COVID-19 cases?[3]
How COVID is diagnosed
Most people know they’ve had COVID because they had a fever or upper respiratory tract symptoms and/or were exposed to an infected person AND had a swab test (PCR or rapid antigen) that detected the COVID virus (SARS-CoV-2) in the upper airway.
At the beginning of 2022, many people with consistent symptoms or high-risk exposures were not able to access PCRs or RATs to confirm their diagnosis, but instead presumed themselves positive and quarantined.
It is possible to diagnose past infection in those who never tested positive. A blood test can look for SARS-CoV-2 antibodies[4] (also known as immunoglobulins). When we are infected with SARS-CoV-2, our immune system launches a precision counter strike by producing antibodies against viral targets, specifically the Spike (S) and Nucleocapsid (N) proteins. COVID vaccination induces[5] a similar immune response against the S protein only. The S antibody “neutralises” the invader by preventing the virus from attaching to human cells.
These antibodies can be detected within one to three weeks after infection and persist for at least six months – potentially much longer[6]. A blood test that shows antibodies to S and N proteins indicates someone has been previously infected. Detection of antibodies to the S protein only indicates vaccination (but not infection).
Read more: How accurate is your RAT? 3 scenarios show it's about more than looking for lines[8]
The problem with antibody tests
Before you rush off to get a COVID antibody test, there are a few notes of caution. There is still much to learn about the characteristics[9] of the immune response to COVID infection. Not everyone mounts a detectable antibody response following infection and levels can decline to undetectable levels after several months in some people.
Because there are other circulating seasonal coronaviruses (such as those that cause the common cold), tests may also pick up antibodies to non-SARS-CoV-2 strains, leading to “false positive” results.
Commercial and public hospital pathology labs can perform SARS-CoV-2 antibody testing, but the interpretation of results should be undertaken carefully.
So, antibody testing should really only be done when there’s a good reason to: say, when confirming past infection or effectiveness of vaccination is important for the current care of an individual. Diagnosing a post-infectious complication or eligibility for a specific treatment, for example. It could also be useful for contact tracing or for assessing the background population rate of infection.
Being up-to-date with COVID vaccinations is still the best protection against severe illness.
AAP Image/Diego Fedele[20]
Read more:
Here's why you might need a 4th COVID vaccine dose this winter[21]
Bottom line
As we enter the third year of the COVID pandemic and given that up to one in three infections may be asymptomatic, it is likely many of us have been infected without knowing it.
If you are experiencing lingering fatigue, brain fog or other symptoms that could be long COVID, you should talk to your GP. Otherwise, knowing our COVID infection status is unlikely to be of much practical benefit. Antibody testing should be reserved for specific medical or public health indications.
Being up-to-date with COVID vaccination is still our best defence against severe illness moving forward.
References^ up to one in three positive people exhibit no symptoms (www.pnas.org)^ highly infectious coronavirus (theconversation.com)^ Has Australia really had 60,000 undiagnosed COVID-19 cases? (theconversation.com)^ SARS-CoV-2 antibodies (www.bmj.com)^ induces (theconversation.com)^ at least six months – potentially much longer (pubmed.ncbi.nlm.nih.gov)^ EPA/Pep Morell (photos-cdn.aap.com.au)^ How accurate is your RAT? 3 scenarios show it's about more than looking for lines (theconversation.com)^ much to learn about the characteristics (www.cdc.gov)^ Seroprevalence studies (www.ncirs.org.au)^ research (www.medrxiv.org)^ plans (kirby.unsw.edu.au)^ fresh seroprevalence studies (www.theguardian.com)^ update local data (academic.oup.com)^ mild (www.ncbi.nlm.nih.gov)^ asymptomatic (pubmed.ncbi.nlm.nih.gov)^ long COVID (www.who.int)^ likelihood (www.nature.com)^ higher viral load (www.cell.com)^ AAP Image/Diego Fedele (photos-cdn.aap.com.au)^ Here's why you might need a 4th COVID vaccine dose this winter (theconversation.com)Read more https://theconversation.com/could-i-have-had-covid-and-not-realised-it-178630
