Corona Daily 343: Think You Already Had It?

Years ago, in my black and white thinking, I had thought a person can have either a virus or antibodies against it, not both at the same time. Now I know I was wrong. From the time the virus enters its host, antibodies can start forming.

In case of Covid-19, a person can start spreading the virus and infecting others even 2 days before he displays any symptoms. Antibodies are not detectable for 6-7 days after the symptoms. In this window of 9 days, two days before and 7 days after, a virus test is critical to detect its presence.  (An antibody test should not be done during this period, because it will be misleading.)

The duration of the presence of virus (meaning positive virus tests) was thought to be 21 to 35 days. The latest CDC guidelines say it is possible for people to get “positive” virus tests up to 90 days. In that time frame, meaning after 7 days from the symptoms and 21, 35 or 90 days as the case may be, an individual can be “positive” on the virus test as well as antibody test.

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As a rule, the sicker one becomes due to an infection, and the stronger the immune response to it, the stronger is the immunity provided. In other words, someone who has been to ICU, battled with Covid-19 and managed to survive may have the longest and most effective immunity. This immunity is specific to Covid-19 and not general immunity as we know it. That person may still be susceptible to a load of other infections.

Some people are convinced they have had a brush with the novel coronavirus. They remember a day or two when they felt feverish in April, or were coughing mildly in June. An article in Washington post called this condition “Thinkihadititis”. These people are happy in the hope they already beat the virus. Well, if the level of immunity is inversely proportional to how ill the virus made you, the joy is misplaced.

A report from China found that younger people had fewer antibodies, and 30% of those sampled had low levels. Some individuals, who had tested positive, either with mild symptoms or were asymptomatic, had no trace of antibodies. This has made immunologists wonder if all such cases may be susceptible to a second infection.

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Antibodies decline, and disappear over time. As one immunologist puts it, if that didn’t happen, there would be only antibodies in our blood with no room for anything else.

How long can neutralizing antibodies last? Covid-19 is not yet a year old, so the data is insufficient to draw any conclusions. Scientists are quoting the data for other coronaviruses in the hope there may be some similarity.

SARS, which triggered an epidemic in 2003, showed survivors maintaining antibodies for 2 years on average. Two milder coronaviruses, OC43 and HKU1, that cause common colds, result in about 45 weeks of immunity on average.

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The important action point from the discussion is that a “positive” viral test in the past does not guarantee continuous or more robust immunity. Particularly the asymptomatic or mild cases should be treated as if they never had the disease, from the social distancing perspective.

Having shared the disappointing news, I will discuss the antibody tests tomorrow.

Ravi