It is easy to get swamped by all the research on coronavirus. New Zealand’s Science Media Centre is keeping track of much of it so you don’t have to. The Research Tracker is prepared by Dr Robert Hickson for the Science Media Centre New Zealand.

Vaccine-related papers

Comparative effectiveness

A Scottish study found that effectiveness of both mRNA vaccines against severe Covid-19 and hospitalisations may initially fall but then stabilise. Effectiveness was 92% shortly after full vaccination declined over 10 weeks to 90%, and then remained at around that level for at least a further 10 weeks.

In contrast, while the effectiveness of the AstraZeneca/Oxford vaccine was similar to the mRNAs’ after vaccination it continued to decline over 20 weeks, reaching 60%. The paper has not yet been peer reviewed.

Another Scottish study found that both the Pfizer/BioNTech and AstraZeneca/Oxford vaccines are 90% effective at preventing Delta-related deaths. For the 40-to-59 age group, the Pfizer vaccine was slightly more effective, while for the over 60’s the AstraZeneca vaccine had a slight edge. No fully vaccinated people under 40 died from Covid. The paper was published in the New England Journal of Medicine

Effectiveness of Moderna vaccine in macaques after one year

In an animal study the Moderna vaccine was effective in controlling infection after one year. There was a delay in the immune response in the lungs, but viable virus was not detected four days after infection. Viral control was less effective in the upper respiratory tract, so there is the potential for transmission.

Eight rhesus macaques were challenged with the Delta variant. The immune responses in the macaques may not be identical to those in people. The paper has not yet been peer reviewed.

Vaccination doesn’t prevent household transmission

A UK study found that vaccinated people had a slightly lower secondary attack rate than unvaccinated people in the same household, but vaccinated people could still transmit the virus. Consistent with papers reported in last week’s Tracker, peak viral loads in vaccinated and unvaccinated people were similar.

However, viral loads in vaccinated people fell more quickly than loads in unvaccinated people. The study also found that faster viral replication lead to higher viral loads, and that viral levels declined more slowly but this wasn’t just due to the viral peak being higher. The study was undertaken when the Delta variant was predominant. The paper was published in The Lancet Infectious Diseases.

Mix-and-match benefit

Having one AstraZeneca/Oxford vaccine dose followed by a Pfizer/BioNTech dose provided better protection against infection than two Pfizer doses. The frequency of infections seen in the mixed dose group was 0.4%, compared with 0.7% for the double Pfizer group.

Sera from those with the mixed doses showed stronger neutralising activity than the double Pfizer. The first and second doses were given about four weeks apart. About 10,000 people received two Pfizer doses, while 2,500 had an AstraZeneca dose followed by a Pfizer. The paper was published in Nature.

Model indicates third vaccine dose reduces infection and transmission rates

A modelling study concluded that a third Pfizer/BioNTech vaccine dose can reduce infections and transmission. The study estimated that a third dose given eight months after the second could boost neutralising antibody levels 25-fold relative to levels for those with only two doses.

Extrapolating from this neutralising antibody level, the model indicated that effectiveness against infection could increase to 87% after a third dose, compared with 60% eight months after the second dose.

In the model the virus’ effective reproductive number decreased by 21% to 66%, depending on vaccine coverage, after the third dose. The paper has not yet been peer reviewed.

Asymptomatic infections after vaccination are underestimated

An Italian study of healthcare workers suggests that infections after vaccination may be more common due to many being asymptomatic. Regular testing of over 2,000 participants who had been vaccinated with the Pfizer/BioNTech vaccine detected 33 asymptomatic cases and eight symptomatic cases.

The asymptomatic cases had IgG levels twice as high as the symptomatic cases, and cleared the virus within a day after their first positive test. In contrast, the symptomatic cases took on average 11 days before they gave a negative result.

Surveillance testing was undertaken every two or four weeks, so the authors note that they probably missed many  other symptomatic infections because of the speed with which the virus was cleared. The paper was published in Annals of Internal Medicine.

Vaccination generates a stronger but more specific immune response than natural infection

An article published in The Lancet Respiratory Medicine reviews the immune responses to SARS-CoV-2 infection and vaccinations. It highlights that the immune response to vaccination, compared with the response to natural infection, is of greater magnitude and higher specificity (although more focused on the receptor binding domain). There is still uncertainty about the durability of immune responses, but the authors suggest vaccines will need to be updated in the “short to medium term”.

Antibody persistence may be shorter in airways than in blood

Antibodies from SARS-CoV-2 infection may not persist as long in airways as in the blood. IgG antibodies in blood were found up to 8 months after infection, but there was a rapid decline of airway antibodies (in mucosa) after 3 months. For those with a prior infection, airway antibody levels were significantly higher after vaccination, compared with levels in uninfected vaccinated people.

The second vaccine dose for uninfected participants generated a larger airway antibody boost than the first, indicating the benefit of full vaccination. The paper was published in JCI Insight.

Vaccinations help reduce risks of a range of Covid symptoms

A study of health records found that vaccination reduced the risk of developing some but not all Covid-19 symptoms after infection. There is good evidence that vaccinations reduce the risk of hospitalisation and severe Covid-19, but the impact on some other Covid-conditions has not been well studied.

This study involved matching vaccinated with unvaccinated but otherwise similar people who became infected. Health records for people infected up to six months after vaccination were included. Most of the data came from the US. Three vaccines were included in the study - the two mRNAs and the Johnson & Johnson/Janssen vaccines.

Two vaccine doses reduced the risk for a wider range of symptoms than a single dose. These conditions include respiratory failure, hypoxaemia,some embolisms, and psychotic disorders. Those under 60 had lower risks for developing Covid-19 symptoms than older people after vaccination.

However, vaccination did not appear to reduce the risk of other conditions, such as long Covid, arrhythmia, sleep disorders, and mood and anxiety disorders. Some individual long Covid-related symptoms (such as myalgia) were less common after vaccination.

The results on the limited effect on long Covid after vaccination disagree with some previous studies. The authors discuss possible reasons for this, noting that their study did not rely on self-selection of, and self-reporting from, participants. The paper has not yet been peer reviewed.

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