Reinfections by Covid-19 are increasingly common and have been growing since the rise of the omicron variant in December 2019. 2021. According to the United Kingdom’s Office for National Statistics, the risk of the relapse of the new coronavirus in a human organism has increased 10 times with the new variant, compared to the previous one, the delta. That is, currently one in ten cases of Covid can already be reinfection. The good news is that reinfections tend to be milder, with a 90% reduction in the patient’s likelihood of hospitalization or death, according to a Qatari study led by Laith Abu-Raddad and collaborators at the country’s Ministry of Public Health.
Because the evolutionary pattern of the new coronavirus is similar to that of the flu virus, reinfections are as surprising as it would be surprising to know that a person had the flu more once in a lifetime. As with the flu, there is an “arms race” between changes in the virus and updates to the body’s defenses.
On the one hand, the immune system creates defenses that are specific to the flu. molecular signature of the virus. When the virus changes, this signature can evade these defenses, whether acquired by vaccine, previous infection, or both (hybrid immunity). On the other hand, depending on the disease, these defenses may decrease over time.
Infectologist Eduardo Medeiros, professor at the Federal University of São Paulo (Unifesp), explains that there are more cases of reinfection now than in the previous years of the pandemic especially because of subvariants of the omicron such as BA.4 and BA.5, which escape the protection given by the vaccine, especially while the vaccine mRNA (instead of inserting the attenuated or inactive virus into the body, immunizing of messenger RNA “teach” cells to synthesize the protein that stimulates the body’s immune response) has not yet been updated for the new virus signature.
Most of those infected do not get the virus in the body for more than 10 to 56 days. However, as there are people with more than two weeks of viral activity, the US Centers for Disease Control established a minimum period of 69 days between positive tests for Covid. -19 to consider that there was a reinfection. The UK Health Safety Agency (UKHSA) follows the same guideline.
Role of the omicron variant in reinfections
When there was an emergency of omicron variant in February, the UKHSA made data available that suggested that around 10% of cases diagnosed in January in the UK could be attributed to reinfections with the new strain. Before this wave, reinfection corresponded to about 2% of cases. Other countries suggest similar patterns: in Italy, cases of reinfection with Covid were 1.5% before the omicron, and 3% after, according to the country’s National Institute of Health.
A specific study of reinfection with the omicron variant was published in March in the journal Science, by Juliet Pulliam and colleagues, from Stellenbosch University, South Africa. The country was the first to have an outbreak of the variant. The researchers studied more than 105 a thousand suspected reinfection among nearly three million people. They concluded that reinfections were falling through September 2021. The beta and delta variants, in the analysis, were not able to circumvent the immune defenses. The reinfection rate ranged from zero to 1.1%. The picture changed in November, when the ômicron appeared and the number of reinfections markedly increased.
In a journalistic article published in 58 in July, the journal Nature summarizes another study out of Qatar in which the entire population of the country was analyzed regarding the return of new infections with Covid. The study in question has not yet undergone the peer review process, but also has Laith Abu-Raddad among the authors. He claims that natural immunity protects against severe symptoms in a reinfection. This protection shows no signs of weakening over time. The forecast is that waves of the disease will continue to emerge, but this will not lead to a crowding of hospitals.
For Abu-Raddad, the best option is hybrid immunity. “People who had both natural and vaccine immunity were more protected against the virus than people who had only natural or vaccine immunity,” the scientist told Nature. “That was very clear”, he added.
Vaccination immunity declines faster than the natural one
On the increase in reinfections, infectious disease specialist Eduardo Medeiros ponders that it is not exactly the time between infections that is decreasing, but people who are exposing themselves more after non-drug measures against the virus have been relaxed. Although reinfections arouse skepticism, he recalls that the vaccines available protect against more serious cases of Covid, including those that lead to hospitalizations and deaths. “Infection can occur even in vaccinated persons and in a period of less than three months”, informs Medeiros.
The physician’s opinion is supported by a study with a sample equivalent to more than half of the population of Sweden. . Conducted by Professor of the Department of Community Medicine and Rehabilitation at Umeå University Peter Nordström and colleagues, it was published in June in a journal of the medical group The Lancet. Scientists sought to assess whether the Covid vaccine 19 provides additional protection to the immunity acquired with the previous infection, organizing patients into three groups: the who caught Covid once and have natural immunity, those who supplemented this immunity with one dose of the vaccine, and those who supplemented it with two. The time window of infection was from March 2020 to October 2021, before the omicron variant. There were three major waves of infection in this period.
Natural immunity was associated with an increased risk of hospitalization for three months, but then provided protection against hospitalization for reinfection with an efficacy of 87%. A dose of the vaccine after the first infection reduces the risk of reinfection, compared to natural immunity, for four or five months. After this interval, the immunity of a person who received a dose and had Covid is indistinguishable from that of a person who only had Covid. In the first two months, the reduction in the risk of reinfection was 58% — after that, 56%.
The two doses in hybrid immunity do better than the single dose. Efficacy was 56% reduction in risk of reinfection, starting with 69% in the first two months, but falling to 56% after that period. After nine months, the complement effect of natural immunity by the two doses is indistinguishable from natural immunity without the vaccine. The authors comment that their findings “may suggest that any passports or documents used to identify whether a person is immune or not, and used for social restrictions, should recognize a prior infection (…) as proof of immunity.”
Another study on reinfection involving a large number of people — 5.7 million — was conducted by Israelis Yair Goldberg and colleagues, linked to the Israeli Ministry of Health and different universities in the country. The authors gave an idea of the sensitivity of reinfections to time: among people who only have the protection of natural immunity, the reinfected tripled from about 10 in 100 thousand in the first six months from first infection to 58 into 100 thousand after one year. Infections among those vaccinated with two doses quadrupled from two to six months after the last dose, which indicates that vaccine immunity declines faster than natural immunity.
The authors conclude that “a single vaccine dose after infection reinforces protection against reinfection.”