Several governments have announced the advance purchase of batches of vaccines against the coronavirus. It is assumed that immunity protects against SARS-CoV-2 infection and that the long-awaited vaccines will be marketed in a few months. However, several studies have just been reported questioning its benefits.
Some say that the population already immunized is greater than the one with antibodies and that, therefore, we are close to obtaining population immunity (estimated at 40-60%). On the other hand, several cases of coronavirus reinfections have been reported after about four months after the first episode.
With this new data, we are going to review the future of vaccines against COVID-19.
SARS-CoV-2 infection, antibodies and immune protection
The journal Cell has published a study by Swedish researchers that concludes that the immune response to SARS-CoV-2 is detected in more than twice the number of patients who have developed antibodies.
It is a “cellular” type response (mediated by T lymphocytes), which is intense and persists for years, unlike the humoral response (directed by B lymphocytes), which usually lasts a few months in human coronavirus infections .
The T immune response can be recognized in patients who have not developed antibodies. These seronegative cases occur, above all, in patients with asymptomatic or mild infections.
In any case, it is an immunity that protects against reinfections, as has been shown in several studies carried out in macaques.
Immune protection has also been confirmed in infections with other pathogenic human coronaviruses, such as SARS-CoV-1 and MERS-CoV. The presence of this immune memory would protect against severe forms of COVID-19 after new exposures to the same coronavirus.
Confirmation of reinfection by SARS-CoV-2
Until recently it was believed that a new positive CRP in patients who had already been cured of COVID-19 likely reflected a laboratory error or a longer-than-usual persistence in viral replication.
This week, however, three PCR-confirmed COVID-19 patients were reported during the first wave of the pandemic in March-April who, after three to four months, had returned to PCR positive.
The first case is a 33-year-old man from Hong Kong, previously healthy, who two weeks ago was examined by PCR for SARS-CoV-2 at the Asian airport upon his return from a trip through Spain via the United Kingdom.
Although he was asymptomatic, the CRP was positive. The patient had already suffered from COVID-19 in March in his country and was recovered. The sequencing of the coronavirus in this second moment showed that it was a different variant from the initial one, with at least 24 different nucleotides that could alter the immunogenicity of the coronavirus.
The mutation rate of the SARS-CoV-2 coronavirus is low, despite being an RNA virus, because it has an error-correcting enzyme in replication. The genetic variability between the first and second viral strain in the Hong Kong patient excludes a reactivation of the original coronavirus.
On the contrary, he favors that it corresponds to a reinfection by a somewhat different strain. In any case, it is important to note that reinfection was asymptomatic and self-limited within a few days, suggesting that previous immunity was sufficient to mitigate the clinical manifestations, although not to abort the infection. Thus, the efficacy of vaccines against SARS-CoV-2 should consider not only protection against new infections but also the clinical severity if they occur.
Following the communication of the Asian case, two other cases of reinfection by SARS-CoV-2 have been reported in Europe. One of them is an elderly man in the Netherlands and the second is a young woman in Belgium. In both cases, the immunity developed after a first episode of COVID-19 did not prevent them from suffering a new infection four months later.
The sequencing of the coronavirus on the second occasion and its comparison with the first viral isolate has confirmed that they are different variants in the three cases. They are reinfections and not reactivations.
In this way, it is confirmed that immunity against SARS-CoV-2 is not durable, as is the case with other coronaviruses, for which the presence of antibodies does not go beyond a few months, gradually disappearing. Reinfections are probably rare, requiring a sufficiently different viral variant and several months of interval between exposures.
With the current information on immunity and vaccines against COVID-19, we can conclude the following:
- SARS-CoV-2 infected patients develop immunity, although the presence of antibodies may not occur in asymptomatic patients or disappear soon in the rest.
- Natural immunity against SARS-CoV-2 is partially protective, that is, it may not prevent reinfections (asymptomatic), but serious clinical forms (COVID-19). This would explain that there is an increasing proportion of more benign forms of the disease.
- Patients cured of COVID-19 (including those with antibodies) should continue to comply with the social distancing measures and the use of masks that are recommended for the rest of the population, since they can be reinfected. Although they have little risk of severe forms, they could transmit the virus to others.
- Vaccines against SARS-CoV-2 should also be considered for people who have previously had the infection. They will reinforce the protective immune response.
Vicente Soriano, Faculty of Health Sciences & Medical Center, UNIR – International University of La Rioja
This article was originally published on The Conversation. Read the original.
