A study published in the National Journal of Community Medicine (NJMC) in October has found that the secondary attack rate (SAR) of Covid-19 infection at households in Patan district stood at nearly five per cent. The findings are in line with a September study by the Indian Institute of Public Health, Gandhinagar (IIPH-G), that recorded SAR in households with a primary infected person at 8.8 per cent in rural Gandhinagar, also in north Gujarat.
SAR is a marker of the susceptibility of spread from a primary source of infection to other specific groups, say within a household, within close contacts, within professional contacts, etc, and helps to understand how specific factors and interactions may be key to the transmission of the infection.
The study undertaken by public health experts at IIPH-G and state health department epidemiologists, which found the SAR in Gandhinagar rural at nearly nine per cent, advocated for more districts and states to come up with similar surveys as “Covid-19 statistics are relatively different” and thus “the SAR prevalence and its factors were also expected to be (different)”. Estimating SAR in various districts and cities, or even wards of cities, is important, it stated.
While the Gujarat-centric studies have indicated a significantly lower SAR, a worldwide meta-analysis of 54 studies with 77,758 participants has estimated overall household SAR to be 16.6 per cent. An Indian Council of Medical Research study on SAR also had pegged Gujarat’s SAR at 7.8 per cent. The study also found SAR across India was in the range of zero to 11.5 per cent, with a national average of 6 per cent.
In Patan, 444 primary Covid-19 positive cases across 389 households were randomly selected for the study conducted between April and August. The study states 1,200 tested Covid-19 positive in the district until August 29, when the state health department had declared 973 cases for the period. Data on these primary cases obtained telephonically found at least 82 of these patients had succumbed. As per the state health department, Patan reported 37 fatalities due to the virus until August 29.
Of the 1,059 secondary high-risk contacts — a near average of three contacts per household — 60 tested positive for Covid-19, which is 5.6 per cent SAR. It also found a majority of the secondary infection patients were men in the 18-60 age group followed by those aged above 60 years. Around 85 per cent of contacts developed symptoms within seven days of contact with the primary case.
Authored by Gujarat Medical Education Research Society (GMERS), Patan, community medicine department associate professors Dr Nitinkumar Solanki and Dr Parul Sharma, and assistant professor Dr Rakesh Ninama, the study hypothesises there can be “many probable reasons for lower SAR”. First, it can be a “milder nature of Covid-19 infection in the home-isolated primary contacts” as around 25 per cent were asymptomatic and others were mildly symptomatic, which may, in turn, reduce the probability of transmission. Among the secondary positive cases, 32 per cent were asymptomatic.
Second, only one-fourth of the 389 households did not have a separate toilet facility while others did, thus making home isolation favourable for the primary cases and reducing the chances of transmission. A third possibility why 94 per cent of the secondary contacts did not develop an infection “in spite of continued contact with the positive case” may be explained by the “possible role of natural immunity or resistance to the disease”.
The study led by IIPH-G was published in September as a pre-print and had relied on meta-analysis to highlight household SAR “varies widely among different populations and ranges from 4.6 per cent to 49 per cent”. The study had looked at rural areas and laboratory-confirmed cases in Gandhinagar and noted that of 386 household contacts of 74 primary cases, 34 contacts developed a secondary infection, and hence SAR was 8.8% in the studied population.
A key difference between the IIPH-G study and the Patan study is that while the former had only considered RT-PCR confirmed cases in its 74 primary cases, the Patan sample population had 66 per cent cases confirmed through RT-PCR, 23 per cent through rapid antigen tests, while another 11 per cent did not know the mode of testing. This, the Patan study states, may have an impact because of the difference in the validity of the two tests used to detect the primary patients.
The Patan study also states “home isolation is a good measure for Covid-19 positive contacts” except for patients above 60 years “who should undergo facility isolation”.
Dr Prakash Patel, assistant professor at the community medicine department at Surat Municipal Institute of Medical Education & Research (SMIMER), said a similar study is currently underway with approximately 400 primary cases. Patel, who is NJMC’s executive editor, said prioritising surveillance among home isolation is a key and does not believe that all patients above 60 years require facility isolation. “What we can do is establish a proper surveillance wherein say very poor households, which do not have an oximeter at home, are prioritised with respect to follow-ups,” he said.
Meanwhile, the international study, ‘Household Transmission of SARS-CoV-2: A Systematic Review and Meta-analysis’, published in the Journal of the American Medical Association (JAMA) Network Open on December 14, also took the Gandhinagar study as part of its analysis. Some significant observations of the study from its over 77,700 samples across 54 studies are that while household and family contacts are at higher risk than others, risks are not equal within the households. “Spouses were at higher risk than other family contacts, which may explain why the SAR was higher in households with one versus three or more contacts,” states the meta-analysis study. The meta-analysis also found household SAR were higher for symptomatic primary cases (14.2 per cent -22.1 per cent) as compared to asymptomatic ones (up to 4.9 per cent), and higher in adult contacts (20.2 per cent -37.1 per cent) than among children (12.3 per cent -21.7 per cent).
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