After researchers in Hong Kong confirmed a person had experienced Covid-19 a second time, reports of similar events have been made by scientists in other countries.
The Netherlands has confirmed four cases of reinfection with genetic sequencing, Belgium one case, the United States has one case and during the weekend a case in Ecuador was also confirmed.
Rather than providing answers, this handful of cases highlights the questions that need answering.
These answers will have implications for pandemic modelling, especially for countries like Sweden, which have rejected lockdown-style measures in favour of herd immunity.
The cases where genetic sequencing has shown reinfection
No case details have been peer-reviewed as yet. The Hong Kong paper has been accepted by a journal for publishing, and the Nevada case exists as a pre-print publication. Dutch researchers are working on papers on some of the cases in Holland. The case in Ecuador was announced by the Institute of Microbiology in Ecuador.
The eight cases where genome sequencing of the virus has confirmed reinfection include:
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A 33-year-old man from Hong Kong was hospitalised with Covid-19 in March. Four months later a swab performed as he returned to Hong Kong from Spain showed he was infected again. He did not have any symptoms;
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A woman in Belgium in her 50s was reported to have a mild case three months after her first infection. Although not published yet, it’s said she didn’t create many antibodies after her first infection;
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An elderly person in the Netherlands with pre-existing conditions was confirmed to be reinfected;
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A nursing home patient in the Netherlands developed a second infection two months after a previous infection;
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In the Netherlands, a man over 60 visited hospital twice with Covid-19 three weeks apart after recovering from his first infection;
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A man in his 60s returned to hospital days after recovering from a first infection in the Netherlands;
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A 25-year-old man from Nevada first tested positive in April. He recovered and had two negative tests. Forty-eight days after his first infection he fell ill again and was hospitalised with pneumonia;
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A 46-year-old man first tested positive in May in San Francisco. In July he returned a negative test. In August, 81 days after his first infection he became sick again and was retested in Ecuador.
What happens after an infection
After being infected with the virus that causes Covid-19, the body’s immune system swings into action, creating antibodies and T cells.
Antibodies are like tiny little shields; they block the spike protein on the virus from latching onto a healthy cell. T cells are more like soldiers and help the body get rid of the virus.
Normally the antibodies and T cells remember the virus. They’re like a pre-trained army. This memory means the next time the virus is present, the defence systems swing into action immediately. A person might not get sick at all, or may have a milder infection.
Immunity to future infections can be affected by the size of the army the body has built. The level of immune response can differ from person to person and this can affect the duration of immunity.
Reinfections were inevitable
The possibility of reinfection has always been expected.
“For any infectious disease there is the potential to get it a second time, and this is more common for some infectious diseases than others. We know that repeat infections occur with other coronaviruses, influenza viruses and other common respiratory illnesses,” said University of Auckland’s Dr Nikki Turner.
Often, older people’s immune response isn’t as strong as a younger person’s, but other factors can come into play.
“For example, people who are on medications that may affect their immune response, or have certain medical conditions that affect their immune response, may have a shorter duration of protection than others.”
Half of the eight cases of reported reinfection involve people 60 or older.
Until more details are published about the individual cases, it remains to be seen what, if any, information gathered from them might help to understand the level of immune response they had after their first infection
The other cases in younger people could show those people – for whatever reason – didn’t have a strong immune response.
This is the first big question where data is missing. What level of antibodies is enough to stop reinfection? The second big question of how long immunity lasts may also be answered by tracking antibody levels and seeing how they change over time.
This is work being done, according to World Health Organisation epidemiologist Dr Maria Van Kerkhove, in longitudinal studies which follow individuals over time.
“From the longitudinal studies that are underway – not all of them are published yet – we do see a strong antibody response that stays at that same level.”
She said in cross-sectional studies, where different people from the same population are followed: “There was some suggestion that there may be a slight decline or waning in immunity.”
Infectious, again
Another important question that appears unanswered from the two papers circulating is whether people who have caught the virus again are infectious to others.
In South Korea, it was thought more than 200 people had been reinfected. This view was later revised and it was thought the reinfections were fragments of the first infection showing up again. These people were not infectious to others.
For these new cases where infection with more than one strain has been proven, it’s not clear whether researchers have looked at this. The Hong Kong study appears not to have measured viral shedding after the second infection. The patient was also isolated.
What does it mean for controlling a pandemic?
In the Hong Kong case there’s a worry: the person had no symptoms. Had his second infection not been picked up through routine border swabbing, he would not have known he had the virus again.
None of the other seven cases reported to date have been in people who didn’t experience symptoms during their second infections.
This could be due to different testing criteria in different countries. In countries where only those with symptoms are tested, there’s a chance asymptomatic cases contributing to virus spread are being missed.
For Sweden, where herd immunity through infection has been pursued, a short time period between infection and reinfection is not good news.
The implications for vaccines
Reinfections aren’t a death knell for vaccines.
Vaccines may also be pivotal to ending chains of transmission through herd immunity, reducing the level of virus in the community. Booster shots are also an option.
University of Auckland vaccinologist Associate Professor Helen Petousis-Harris didn’t jump to conclusions after the Hong Kong case.
“This finding does not necessarily have bearing on the vaccines under development because they are not mimicking natural infection. In the case of this coronavirus, natural infection actually interferes with the immune response, something that vaccines will not have to contend with.”
