DOC­TOR'S NOTE with Dr Amir Khan | AL JAZEERA

● How past ex­po­sure to oth­er coro­n­avirus­es can stop peo­ple from catch­ing COVID-19 even if they live with some­one who has test­ed pos­i­tive. Plus, how to recog­nise the symp­toms of a blood clot ●

(AL JAZEERA) — Many of us will be fa­mil­iar with this sce­nario: Some­one we know has test­ed pos­i­tive for COVID-19 but de­spite liv­ing with that per­son, one or more mem­bers of their house­hold con­tin­ue to test neg­a­tive. How is this pos­si­ble? We know that SARS-CoV-2 is main­ly spread through air­borne par­ti­cles. We have al­so seen how quick­ly it can spread through com­mu­ni­ties. So, how then can the peo­ple breath­ing the same air as those in­fect­ed, re­main un­in­fect­ed?

Ac­cord­ing to a new study, the an­swer may lie in peo­ple be­ing ex­posed to oth­er coro­n­avirus­es in the past that en­abled them to make mem­o­ry im­mune cells, called T cells, which of­fer pro­tec­tion against the SARS-Cov-2 coro­n­avirus. The term coro­n­avirus is a broad one that en­com­pass­es a num­ber of virus­es in ad­di­tion to the one that caus­es COVID-19. Many of the com­mon colds we see are caused by virus­es that be­long to the coro­n­avirus fam­i­ly. The study sug­gests that mem­o­ry T cells cre­at­ed by ex­po­sure to these virus­es could be why some peo­ple test neg­a­tive for COVID-19 de­spite liv­ing with some­one who has test­ed pos­i­tive. Al­though the dif­fer­ent coro­n­avirus­es cause dif­fer­ent ill­ness­es and the virus­es be­have in dif­fer­ent ways, they do have some shared char­ac­ter­is­tics that group them to­geth­er in­to the same fam­i­ly. Their shared struc­tur­al sim­i­lar­i­ties en­able im­mune cells that recog­nise one type of coro­n­avirus to recog­nise an­oth­er type.

The study fol­lowed 52 COVID-19 house­hold con­tacts. Its aim was to cap­ture the ear­li­est point at which these peo­ple de­vel­oped an im­mune re­sponse to the SARS-Cov-2 virus. Blood was test­ed for im­mune cells be­tween day one and day six of when their house­hold con­tact test­ed pos­i­tive for COVID-19. They found high­er lev­els of mem­o­ry T cells in the sam­ples of those who test­ed neg­a­tive on a PCR test for COVID-19 than those who test­ed pos­i­tive.

The au­thors be­lieved the lev­els and speed at which the mem­o­ry T cells be­came ac­tive af­ter be­ing ex­posed to their house­hold con­tact of COVID-19 sug­gests these im­mune cells were pre-ex­ist­ing from pre­vi­ous coro­n­avirus in­fec­tions, and not from the cur­rent ex­po­sure to the SARS-Cov-2 virus. This is why this group of peo­ple did not test pos­i­tive on a PCR test. They be­lieve these pre-ex­ist­ing T cells trig­gered an im­mune re­sponse that quick­ly dealt with the SAR-CoV-2 virus be­fore it could in­fect the in­di­vid­ual and show a pos­i­tive PCR test. They did al­so find these T cells in the group that test­ed pos­i­tive, but the lev­els were not as high.

The T cells that they mea­sured not on­ly tar­get­ed the spike pro­tein part of the virus but al­so the nu­cle­o­cap­sid, the core part of the virus that stores its ge­net­ic ma­te­r­i­al. They con­clud­ed that fu­ture vac­cines against the SARS-CoV-2 virus may need to in­duce a wider im­mune re­sponse than many of the cur­rent ones, which cre­ate an­ti­bod­ies to the spike pro­tein part of the virus on­ly. Hence, they are more like­ly to re­main ef­fec­tive even if new vari­ants arise with new mu­ta­tions.

As a re­sult, the next gen­er­a­tion of COVID-19 vac­cines may tar­get core coro­n­avirus pro­teins as well as the out­er spike pro­tein.

Read­er’s Ques­tion: How to spot the signs of a blood clot

An illustration of a blood clot with a web of blood vessels showing the inside of the front facing blood vessel showing something blocking the flow of blood cells.[Muaz Kory/Al Jazeera]

There are sev­er­al dif­fer­ent types of blood clots and de­pend­ing on where they oc­cur in the body, their symp­toms will vary. One of the most com­mon types of blood clot to get is a deep vein throm­bo­sis (DVT). A DVT is a blood clot in a vein, usu­al­ly in the leg and can po­ten­tial­ly be quite dan­ger­ous.

A blood clot is a thick gelati­nous mass of blood cells and clot­ting pro­teins that can block a blood ves­sel and stop oxy­gen and food from get­ting to vi­tal or­gans and tis­sues.

Blood is nor­mal­ly a vis­cous flu­id that likes to flow freely along blood ves­sels. Any­thing that af­fects the flow of blood through blood ves­sels can in­crease the risk of a clot form­ing. Things that can af­fect blood flow in­clude any­thing that af­fects the in­side wall of blood ves­sels such as fat­ty de­posits, swelling or in­flam­ma­tion from smok­ing, ir­reg­u­lar heart rhythms and pe­ri­ods of im­mo­bil­i­ty that slow blood flow down. Be­ing preg­nant, hav­ing can­cer and be­ing over­weight al­so in­crease a per­son’s chances of de­vel­op­ing a clot.

A blood clot in the leg can present with one or more of the fol­low­ing symp­toms:

●   Throb­bing or cramp­ing pain in one leg (rarely both legs), usu­al­ly in the calf or thigh

●   Swelling in one leg (rarely both legs)

●   Warm skin around the painful area

●   Red or dark­ened skin around the painful area

●   Swollen veins that are hard or sore when you touch them

The main con­cern with clots in the legs is that there is a small chance they can break away and trav­el to the lungs and ob­struct the vi­tal blood ves­sels there. This is called a pul­monary em­bolism.

DVTs are usu­al­ly di­ag­nosed on a scan and treat­ed with blood-thin­ning med­ica­tion. Some­times, this is giv­en as in­jec­tions, but for most peo­ple, it will be giv­en as tablets that they will have to con­tin­ue to take for sev­er­al months or longer.

The best way to re­duce your risk of de­vel­op­ing a DVT is to keep a healthy weight, keep ac­tive and keep hy­drat­ed.