Covid-19 Antibodies Test Positive! How Happy Should You Be?
Antibodies save lives. In this pandemic, antibodies to Covid-19 may be your most valuable possession.
Our immune system is a very sophisticated and effective defense system against a world teeming with germs and diseases. When a germ enters the body, immune system cells produce antibodies; these protect against the germ if it comes by again.
Antibodies against any disease are attained by having (and surviving) the disease or by specific vaccination. Some people just have them; presumably, they had an infection so mild that they did not recognize it.
In this pandemic, antibodies against the Covid-19 (also known as SARS-CoV-2) virus are a focus of attention. Your doctor may advise you to have an antibody test; you may choose to get it done on your own.
What Does a Positive Test Mean?
A positive antibody test means that your body has produced antibodies to the virus.
You have been exposed to the SARS-Cov-2 virus at some time in the past. Your immune system did its job and your bloodstream has antibodies.
It does not mean that you have a Covid-19 infection at present.
Is an Antibody Test Diagnostic of Covid-19?
No, these tests show past exposure to the virus. They do not reliably diagnose a current infection. Antibody levels are not used to diagnose active disease.
If you have symptoms suggestive of Covid-19, talk to your doctor. She will advise a nucleic acid amplification test (NAAT) or a reliable antigen test.
Antibody Levels After Infection
IgG antibodies to Covid-19 appear a week to two after the infection, reach a peak at about 20 days, and then decline. The decline is rapid in the first 30 days, but then slows.
Patients with a severe infection have high levels of antibodies. However, the fall in levels is also rapid in these people. Sixty days after infection, all recovered patients have almost the same level of antibodies.
But is Having Antibodies Complete Protection Against Covid-19?
Four issues need consideration:
- Are the antibodies protective?
- Which antibodies are protective?
- How long does protection last?
- Are laboratory results dependable?
Are Covid-19 Antibodies Protective?
Yes, they are.
In a study of health care workers, Lumley et al. found that the risk of infection with Covid-19 was eight times lower in those with antibodies to the spike protein than in those without the antibodies. They found that health care workers with anti-nucleocapsid IgG antibodies also had protection against Covid-19.
Harvey et al. studied a database with over three million people with and without antibodies. They found that the infection risk was only a tenth in those with antibodies, compared to those without antibodies. This tenfold lower occurrence of Covid-19 disease in people with antibodies is similar to the protection found with the mRNA vaccines.
The Wisconsin School Retreat Outbreak
In July 2020, a Covid-19 outbreak occurred at an overnight retreat for boys. The retreat included 152 boys, counselors, and other staff. By the end of the outbreak, 118 (78%) had tested positive.
Twenty-four participants were antibody-positive before the camp started. Despite masks not being worn, none of them got infected.
Which Antibodies are Protective?
Laboratories commonly measure two types of antibodies: anti-nucleocapsid IgG, and anti-spike protein IgG.
The Covid-19 virus needs the spike protein to attach to host cells in our body. Antibodies against this protein should be protective against cell invasion by the virus.
However, both the antibodies are protective. People with either of these antibodies are protected against infection with Covid-19.
The other types of antibodies are IgM antibodies. These appear early in the infection, but do not persist.
How Long Does Protection Last?
The disease itself is so new that we know little about its long-term behavior. As of this writing (March 2021), we know that antibodies are associated with protection for six months, at least.
Most of the evidence so far available is with naturally-occurring antibodies. That is, antibodies in people who have had Covid-19 disease or asymptomatic infection.
Antibodies generated by vaccination are still being studied. Their persistence in the blood, and the protection achieved, will be known in the next few months. Answers that should be forthcoming:
- The degree of protection from infection
- The degree and duration of protection from symptomatic disease
- The degree and duration of protection from hospitalization and death
- Whether people with antibodies can spread the disease.
Are Laboratory Results Dependable?
On the whole, yes. However, in a small proportion of people who take the antibodies test, there can be errors.
False-positive. There are no antibodies, but the test is positive. This could be because of cross-reactivity with other coronaviruses. False positives are rare; most antibody tests for Covid-19 are highly specific.
False-negative. There are antibodies, but the test is negative. This could be because of very low levels of antibodies. The sensitivity of approved Covid-19 antibody tests vary from 87% to 94%.
Antibody disappearance. A person who has had the disease tests negative because the antibody levels have declined to undetectable levels.
Many of the fingerprick self-tests sold over the counter have been found to be unreliable. If you want to know whether you have antibodies, going to a good laboratory is best.
The Other Immunity — T-Cells
Apart from antibodies, our body also generates immunity to infections by cellular killing of disease-causing agents. These cells, called T-cells, are probably involved in the immunity to Covid-19 that results from an infection, but not from vaccination.
Cell-based immunity is harder to measure than antibody levels. Its potential contribution to protection from Covid-19 complicates the interpretation of antibody levels.
Is Protection Complete?
Protection is relative rather than complete. Most people who have antibodies do not fall sick, even with significant exposure to infected people.
A small number of people with antibodies do get re-infected. However, the disease is usually mild in these people.
What does it Mean for You?
Now that you have antibodies and have capable protection against infection, should you discard all precautions?
Not a wise thought at all. Re-infections do happen, though they are rare. Avoiding crowded places and unnecessary travel are recommended for everyone at present. And keep your mask on when outside your home. If you want to eat out (when masks have to be off), you should still opt for open-air places.
And though you have protection against Covid-19 disease, you can still carry and transmit the virus. To the people you meet: family, friends, colleagues, and others.
So What Can You Do?
Groups of people who are all vaccinated can meet in indoor spaces without a mask.
As more and more people get vaccinated, parties at homes may be possible again. It’s a sad thought, but you may have to draw up your guest list based on antibody test results or the vaccination status of your friends.
Can You Help Others?
This was the great hope a few months ago: people who had survived the disease could help seriously ill patients by donating plasma. The antibodies in the plasma would be effective in treating the disease and would save lives.
That didn’t really work out. Several trials have shown indifferent results, and plasma therapy for Covid-19 has been almost given up.
You can help friends and family who are isolating because of travel or exposure to the disease. People with proved SARS-CoV-2 infection and mild symptoms also isolate at home. Since you are relatively safe, you can help them by doing their shopping for them, and delivering food and other essentials to their door.
However you got them, if you have antibodies to SARS CoV-2, you are fortunate.
Staying safe at home isn’t possible permanently, and exposures happen in the workplace and other places. If you have antibodies, you are relatively safe from the infection and almost entirely safe from severe disease, hospitalization, and death.
Relatively safe is the important concept here. The protection by antibodies is not complete, and we don’t know how long it persists.
It’s best to stick to the good habits: avoid crowds and travel, wash hands frequently, wear a mask outside the home, and keep your guard up.
Harvey et al. Association of SARS-CoV-2 Seropositive Antibody Test With Risk of Future Infection.
Lumley et al. Antibody status and incidence of SARS-CoV-2 infection in health care workers.
Pray et al. COVID-19 Outbreak at an Overnight Summer School Retreat – Wisconsin, July-August 2020.
Yamayoshi et al. Antibody titers against SARS-CoV-2 decline, but do not disappear for several months.