Canada has seen more than three million recovered cases of COVID-19 since the start of the pandemic. While overcoming symptoms and returning to a normal state of health may come with a sigh of relief, experts say previous infection doesn’t necessarily prevent someone from contracting the virus again.
According to Dr. Sumon Chakrabarti, an infectious diseases physician with Trillium Health Partners in Mississauga, Ont., the possibility of being reinfected with COVID-19 has existed throughout the pandemic. It’s also possible to be reinfected with the same strain of COVID-19, said Dr. Martha Fulford, an infectious disease specialist at McMaster Children's Hospital in Hamilton, Ont. However, the chances of severe disease in this case are lower, she said.
“We know that you can get reinfected with the same strain, but…our immune system works to build antibodies to the strain that you're first exposed to, and those antibodies are usually very good at preventing symptomatic disease again,” she told CTVNews.ca on Wednesday in a phone interview.
Reinfection from respiratory viruses in general is quite common, Chakrabarti said.
“Respiratory viruses are known to have some level of immune evasion,” he said in a phone interview with CTVNews.ca on Wednesday, pointing to their ability to dodge immune responses. “SARS-CoV-2 being able to reinfect you is no different than what other circulating respiratory viruses do.”
What’s important to note, however, is that the vast majority of reinfections are likely to be mild, Fulford said. When someone discovers that they’ve been reinfected with COVID-19, the first thing she asks is whether or not they’re feeling sick, she said.
“Respiratory viruses circulate every single year and people do get reinfected, the key is they don’t get particularly sick,” said Fulford. “If somebody is getting reinfected [with COVID-19] and getting super sick and landing in hospital, I'd be looking really hard at that person's immune system for a compromise…that’s very unusual.”
Both experts describe mild illness as similar to a cold, consisting of symptoms such as a runny nose, cough, sore throat and headaches that span across a few days. Mild cases of COVID-19 are also unlikely to require hospitalization or ICU admission, Chakrabarti said. The reason why most cases of reinfection are likely to result in mild disease is due to previous exposure to the virus, he said, as the body is more easily able to recognize it and has already developed antibodies that target it specifically.
As part of the body’s first line of defence against disease-causing organisms, neutralizing antibodies are produced in an effort to protect against the virus, Chakrabarti said. But the immune system also generates a memory response, otherwise known as immunological memory. This is the immune system’s ability to quickly identify a foreign substance it has encountered before. This triggers an immune response that involves not only neutralizing antibodies, but also T-cells and B-cells that help protect the body.
This concept also applies to those who have been reinfected with SARS-CoV-2, particularly if it’s the same variant, said Fulford. Vaccines also play a role in reducing the severity of disease, she said.
“Your immune system would be targeted to that strain,” Fulford said. “You may still get reinfected, but each time you're less likely to have severe illness.”
REINFECTION WITH OMICRON
An analysis from the United Kingdom’s Office for National Statistics indicates that the risk of reinfection was 16 times higher during the Omicron-dominant period compared to when Delta was circulating as the dominant variant. The time frame outlined as Omicron-dominant spanned from Dec. 20 to Jan. 9, while the period of time dominated by the Delta variant, as per the analysis, was May 17 to Dec. 19.
A more recent study from the same statistical institute also shows that a greater proportion of people infected with the Delta variant saw symptoms with their second infection, compared to the number of people who experienced symptoms during their first. Meanwhile, about the same number of people infected with Omicron experienced symptoms during both infections, but no data was provided on the severity of illnesses. Many researchers, including those with the Centers for Disease Control and Prevention in the U.S., are performing ongoing studies on the severity of reinfections.
When it comes to COVID-19 reinfection, it's also important to consider waning immunity, said Chakrabarti. Antibody levels will always be highest in the first two to three months after infection or vaccination, he said, before they begin to drop. As a result, it’s possible for reinfection to occur after about three to six months. What remains, however, is an ability to continue to protect against serious disease, particularly if someone is vaccinated. Previous infection as well as vaccination are helpful in preventing against future severe disease, he said.
“As a natural consequence, those circulating neutralizing antibodies do eventually drop off with time,” Chakrabarti said. “But the memory of that virus will still be there so if it ever comes back to see you again, it might be able to get past the frontline neutralizing antibodies, but once it gets into the cell and the memory system kicks in, then it's not going to go very far.”
Many factors play a role in Omicron’s seemingly higher risk of reinfection, said Chakrabarti, including its enhanced ability to evade immunity from past infection or vaccination. High transmissibility also contributes to an increased ability to reinfect, he said, particularly in those who previously had Delta.
“It's not usual for, in the middle of a season, to all of a sudden get a massive variant replacement,” he said. “It just so happened that because Omicron suddenly expanded so quickly, you saw people that got [COVID-19] again within three months.”
Omicron is also believed to replicate faster in airways as opposed to lung tissues, as demonstrated in several recent studies. According to Chakrabarti, this is another reason why the variant is more likely to reinfect patients than previous strains of COVID-19.
“It tends to mainly copy itself in the upper airways, outside of your lungs,” he said. “That's closer to the outside environment and makes it easier to spread. That, plus the ability to evade the frontline immune response, is probably why [Omicron] is able to much more easily reinfect you compared to Delta.”
All of this stems from the virus’ ability to mutate and evolve in order to survive, said Fulford, something common among viruses in general.
“We know that viruses change and alter all the time because their job is to…carry on being a virus,” she said. “The strains that escape our immune system are the ones that are going to be the next wave of the virus and this is true every single season of all viruses.”
COVID-19 TESTING
Aside from any obvious symptoms, someone may discover they’re reinfected with COVID-19 after taking a polymerase chain reaction (PCR) test. However, Fulford said these tests can remain positive for up to 12 weeks after the onset of illness. Therefore, there’s a chance that if someone is testing positive after only two or three months, it may be falsely attributed to residual virus from the initial infection, she said.
“The dilemma is that if I then get a cold in two months and somebody uses a pharyngeal swab on me… if it picks up virus, it may be dead virus from the infection from two months ago,” said Fulford.
While a PCR test can determine whether or not someone has COVID-19 by testing for genetic material, it cannot tell whether the virus is still viable or if a person is still infectious. It’s for this reason that it’s generally not recommended to retest a patient with a PCR test within 90 days, she said. According to the Public Health Agency of Canada (PHAC), it’s possible for PCR test results to remain positive or fluctuate for weeks or even months after they were first conducted. Positive results obtained within three months of a previous infection might not actually represent a true reinfection, reads PHAC’s website, and it is not yet clear how soon after a COVID-19 diagnosis that reinfection can take place.
“Retesting a PCR too quickly is uninterpretable, that’s true of all PCRs,” said Fulford.
Results displayed using rapid antigen tests (RATs), on the other hand, are not expected to remain positive for as long, Fulford said, given that these tests target specific proteins on the surface of the virus as opposed to viral genetic material. According to PHAC, the ideal frequency of repeat testing using an RAT has not yet been determined.
According to both experts, exposure to the virus at some point is inevitable. However, the mild nature of most infections, particularly for those who are vaccinated, can provide a sense of hope, said Chakrabarti. Still, COVID-19-related hospitalization remains a concern for many Canadians, particularly those who are older in age or immunocompromised, he said. According to data compiled by CTVNews.ca, there are currently 6,907 patients in hospital with COVID-19 and 896 COVID-19-related ICU admissions. While hospitalizations across the country began to rise at the end of 2021 and into the new year, many provinces have seen numbers drop as of early February.
“The best thing for us to remember is that [infections] are overwhelmingly, at this point, mild, especially for vaccinated individuals, and your immune system has been dealing with this for a lot longer than you’ve known about COVID,” he said.
“Being exposed to a virus and developing antibodies to it and not getting as sick the next time you're exposed is what human beings and viruses have been doing for millennia,” Fulford said.
